I went up to Matlock Combat Games on Saturday with friends who are regulars, really enjoyed it. Large site, well organised, actively marshalled and lunch with the price. £25 non-member walk on.
Black Dagger near Chesterfield is fun as well. Small site and honestly a snipers paradise but friendly and fun.
Bassett's Pole paintball do airsoft as well now. Very slick and professionally run, less friendly than the above.
Combat Ready Airsoft aka Crafty's in Derby I've tried to go to but they were full. Friends who made it in and have been before have had generally pretty poor experiences (lots of cheating, ineffective marshalling) to the point of leaving on several occasions. Word from other players has been pretty bad too so I'm avoiding, your mileage may vary of course.