I am planning to visit Maldives in couple of weeks.I need a cheaper wide angle lens extension or wide angle lens itself which could fit my Canon EOS 500 D.I would like to capture whole beach in my lens.Please ponder your suggestions which is available in UK.For your info I already have a canon 18-55 mm Kit lens, Canon 70-300mm lens, Canon EF F-1.8 USM lens.
There are no lenses in your price range.
The adapter you linked to is like putting a cheap magnifying glass in front of your camera (only in reverse, it's making things smaller instead of bigger) - those things are know for low image quality, it's very likely to cause a lot of distortion and I bet that if there's a lot of sunlight (like on a beach) it will cause a lot of lens flare.
Just for reference that cheapest lens that is significantly wider than your kit lens (at least today on Amazon) is the Tamron 11-18mm at 280 quids used.
But, you still have some good options that don't require spending lots of money:
First, 18mm is pretty wide and you can take great beach pictures with your kit lens (hint: if you want a wider image you can also just move back).
Second: you can do a panorama - take multiple image and stitch them together on your computer, there are some really good free programs you can use for this - that will effectively simulate a much wider lens (and it doesn't cost anything).
If you need cheap lens, and you have already 18-55 mm Kit lens, you no need other glass. If you need "whole beach" on picture, try panorama mode and/or just stick few photos together. This is easiest and best way my opinon. Better invest in good polarizer and UV filter.
Here is my sample. Not beach of course ;) But vide view. 2 photos used, but can be more.
This is probably the least expensive ($479 B&H) super-wide zoom lens you will find. Sigma makes two other lenses one of similar focal length range 10-20mm constant f3.5 aperture and also the really well reviewed 8-16mm which is probably the widest non fisheye lens available for APS-C sencor cameras. (both are more expensive, the 8-16mm is $699 at B&H)