Both of your tripod selections look good to me. If you can live within their limitations, they should work fine for you.
However, both of them are large and heavy enough that they cannot be called "convenient." They're not the small but highly limited sort of tripod you can tuck into the corner of a camera bag.
I find this sort of middle ground uncomfortable. I'd rather:
Pick a cheap, highly limited tripod that will serve some limited purposes, but which will likely fail me in the future; or
Spend the money on one big and strong enough that it will always always always work.
The Manfrotto 055 and 190 series leg sets weigh about 2-3× that of the tripods you're looking at after you add a head, and they are bigger. Yet, they're still about as carry-able as the ones you're looking at, unless you've got a strong weight or size constraint. If you can tolerate taking one of the small ones you're looking at, you can probably tolerate a "real" tripod, too.
Tripods are inherently bulky, awkwardly-shaped objects. Ideally, we want them to outmass the camera by several times to get a mass dampening effect. We can compromise on all of this, but we lose benefits with each compromise.
For myself, most of the time I use a tripod it's either in the studio or because I'm going on a car trip. In either case, there isn't much of a real limit on weight or bulk.
Maybe for you airplane trips matter more, so you have to account for the size of your checked bag. In that case, you have a hard limit that will constrain your choices. This is a good thing. It means you can eliminate huge swaths of your choice space.
One other consideration you don't seem to have thought about: neither of your choices apparently allows you to change out the head. You should spend more time thinking about the head than the legs; the proper head for you is a functional and esthetic choice that depends on your use cases and personal taste, whereas the choice of leg set is a purely practical matter that anyone competent could advise you on.1
A nice head can turn a barely adequate leg set into a good tripod. A miserable head can keep a wonderful leg set in the closet.
Is saving £110 (~$173) buying a tripod with only 1.5kg payload false economy?
Put it this way: I still have and use the first tripod I ever bought, over 2 decades ago now, a Bogen 3001 leg set with a Bogen 3030 head.2
I bought that tripod at full retail price, yet it's amortized out to about the cost of an upscale hamburger per year. I broke it once, but that's only because I tried to use it as a crowbar. I was able to repair it for a few dollars worth of parts, because I chose one popular enough that its spare parts are still in production.
That's good economy.
I've taken this tripod on 2-3 mile hikes. With the optional shoulder strap, it's light enough to go anywhere, as long as you don't have some strong constraint on size or weight. I don't know that it would fit into today's cramped airline bag limits, and it's not small enough that I'd be willing to strap it to a backpack for a week-long hiking trip. Outside such limits, it's small and strong enough for almost every normal photographic purpose.
I also have a larger and heavier tripod that I prefer to use because it gets the viewfinder to eye level without raising the center column. This is the tripod I take on car trips, but I have a strap for it, too, and have also taken it on long day hikes without too much hassle.
You definitely know it when you're humping either of these around. But, I suspect that is true of the smaller tripods you're looking at, too. Again, good tripods are inherently bulky, awkward, and heavy. When you break down and decide to carry one, you want it to have the mass and stiffness it needs to do the job you bought it for.
Would it be better to go with the cheaper one first and then upgrade when I actually find a need to do so?
Over the decades, I've bought a bunch of small, cheap tripods. I've stayed out of the middle ground you're looking at, so maybe it this observation doesn't apply to you, but every single one of those tripods has ended up given away, thrown away, stored away, or spirited away. I have never replaced one of these with a like-kind tripod. Instead, I just get repeatedly suckered by the dream of a small tripod that will really work this time. Every time, the spiffy wee little wonder has failed me.
Many things sold as tripods are actually just tripod-shaped objects. They look like tripods, but they are not tripods. We have definite things in mind when we buy a tripod, and an object that does not do these things is not a tripod. It is a TSO.
I've never liked the old saying about a poor carpenter blaming his tools. It presumes that there is a floor below which "tools" cannot go. In today's marketplace, there is no floor.
You can go out today and buy a plastic hammer, and it will be absolutely useless for building a fence. Yet, if you use that hammer for its intended purpose — pounding tent stakes into the ground — it's adequate. It's better than a rock: it weighs less, it's safer to use, and you can't always find a suitable rock anyway.
That's where you are with these tripods. They're far better than nothing, but they may not be suitable for the purpose you will put them to. Only you can make that call.
should I be worried if I have a lens that gets close to the 920g maximum limit?
Not really. Manfrotto tripods are quite well engineered. Manfrotto isn't going to specify a limit right at the margin of mechanical failure.
There are several signs that you bought a tripod that is too small or too weak:
The head lock slips even with reasonable force applied to the locking knobs/levers. When you find yourself denting your fingers in order to tighten the head enough that you don't dump the camera as soon as you let go, it's time to look at something sturdier.
The legs shake enough due to mirror slap, wind, ground rumble, or the shutter button press that it affects photo quality.3
You get it out into the field and find that it isn't tall or versatile enough.4
Bottom line, you shouldn't worry overmuch if you decide to go ahead and get one of the small tripods you're looking at. If it later turns out that you have to get something bigger, a small, quality second tripod is a handy thing to have. You can use it for airplane trips, long hikes, or make it your small-system tripod. I've used my smaller tripod as an extra light stand, to hold the SPL meter when calibrating my home theater, etc.
What dimensions does it need to have? Does the center column have to be reversible? Do you need carbon fiber, or will aluminum suffice? Etc.
This combo is also known as the Manfrotto 190D leg set and 141RC head.
Bogen Imaging distributed Manfrotto's products in the US under the Bogen name with a different model numbering scheme for decades. Several years ago, Bogen got absorbed into Manfrotto's parent company, so now the products are branded Manfrotto in the US, too.
The Bogen 3030 head isn't available any more, but the new X-Pro 3-Way head is a superior replacement in just about every way. The retractable handles reduce bulk, the bubble levels are often handy, the adjustable tension is quite useful, and the QR locking mechanism is easier to purposefully disengage without being easier to accidentally disengage.
The primary downside of the X-Pro 3-Way head is that it could not survive being used as a mace to fend off Uruk-hai, whereas a 3030 would. Just clean the gore out of the QR plate hollow and you can return to your composition. The X-Pro will merely survive normal photographic abuse.
The 804RC2 is more in the style of the old 3030, if you anticipate that your tripod will need to double as a +2 Mace of Smiting.
You test this with long exposures of a good photographic resolution test chart. I like this one. Print it onto tight-weave matte paper, tape it up on a wall, light it well, and shoot it at increasingly long exposures at f/8 and ISO 200. Do this test using whatever shutter release, remote control, or self-timer setting you would normally use while the camera is on a tripod.
If you expect to press the shutter button by hand while using this tripod, do the test that way instead. You want to mimic real shooting conditions with this test.
If the test shots look fine at, say, ⅛ second but start getting blurry below that, you know the tripod isn't doing its job, which is to keep the camera still.
If you get blurry shots under conditions you expect to shoot under, you have to ask yourself something: Didn't you buy a tripod to prevent this?
I bought a tripod once that was decent while completely folded, but when extended to its full 4½ foot height, you could literally see it un-flex when you took your hand off the camera.
Useless? No, not at all. It's fine as a tabletop tripod, and even when extended it's better than duct-taping the camera to a tree. But in the end, it's really just another TSO.