http://www.english-country-cottages.co.uk/

thats who we use when going down to devon, dorset, cornwall etc. Wouldn't call it cheap but the some of the cottages we've been to are fantastic, they have very good facilities (swimming pool, snooker, sometimes tennis courts) and the layout of the rooms are well suited for younger kids.
Staying at a complex with good facilities is great if you plan staying in for a couple of days instead of going out all the time, but if your planning on going out each day then your best bet is to just stay at a basic cottage complex and save some money

Hope that helps