Gunman Tuddenham is a excellent site with a wide variety of combat scenarios (and I hear they've been working to make it even better since I was last there). Very large, and the urban action in the village is great fun. It's a nice blend of CQB and long range stuff, so there's a role for everyone.
I had some bad experiences when I went, being new to airsoft (hire guns were expensive and not a good quality, plus I had a tac-vest stolen and the marshals weren't particularly willing to help), but it's definitely somewhere I'd consider going back to now I have my own kit and know what I'm doing.