Not sure what's commonly available in your area in the UK, but if you do a search on this forum for "which cat", "best cat", etc., you'll find lots of information where others have gone before.

That being said, one critical issue is whether you plan to sail solo or with crew (how many?) as a rule. That will basically answer the question of bigger/smaller. Other than that, I would start scouting what's available in your area that fits your budget and then do a search on this forum for those particular boats you encounter.