One of the biggest barriers to wildlife photography as I see it is the requirement for fast, long expensive lenses. I have in the past used a teleconverter to get my 300mm lens up to 600mm but even that seems barely adequate for anything except particularly large or close subjects.
If you are properly prepared, e.g. scout your subject in advance for several days (or weeks), wear appropriate camouflage, find downwind vantage points etc, can you actually offset this need for expensive glass by allowing yourself to get closer and thus manage with, say, a 300mm lens? Or is there still a longer minimum focal length you should be working with?
Please bear in mind that I am referring to UK wildlife, which generally consists of small mammals and birds - no elephants here!