There are plenty of places to look. When I was trying to buy my aircraft, I used the following websites (I can't recall the URLs, but if you put those terms into google, you should find them). I hope these get you started. I'm sure there'll be more suggestions along in a minute. You can also check out Flyer and Pilot Magazine and you'll probably find a copy of GA buyer lurking around your nearest flying club.
Avbuyer
aeroclassifieds
ukga aircraft classifieds
pilotweb classifieds (from Pilot magazine)
justplanetrading
gtaviation
global plane search (but remember to put in 'UK' in the search fields unless you are prepared to travel)
My strong advice is to get a trusted licensed engineer to complete a full survey before you buy. The comment previously made to do some footwork and perhaps talk to other people at the club is also sound advice.
Good luck.