if I were you I'd take a look at Owners Direct -
although there is something for everyone to suit all tastes and budgets, if you look for something that has a tourist board rating (5 is very good / 5 gold is the best you'll get) you'll know you've got a quality place to stay . in my experience the owners take far more care than the agencies, and the little extra touches / attention to detail are what makes owners places really special .
you can select apartments / cottages / farmhouses / castles, etc to suit all over the UK / rest of world - and basically search for whatever you're looking for - ie. is wifi important, or a sea view, or a log cabin , cot , Jacuzzi , etc. etc. ? there is something for everyone !