Here is a great site I go to that is very close to Buckinghamshire, however it depends where in bucks you are. They have a site in Dunstable and one in Wycombe.
The site I have linked you to is a CQB site I have been to many times that is great nice mix of indoors and out. The Wycombe one is a woodland site that is apparently really good although I haven't been there myself.
There is another one on the way to Watford which is ok ish called Red 1 airsoft. Havent been there since I went to Xsite though as that was much better in my eyes.