Welcome LibertyBell, There are certainly plenty of old RAF airfields to choose from if you are basing your circuit how it looked in it's heyday.
Alas your circuit wouldn't be viable these days, Theres a pic of how
http://local.live.com sees it now a days, at the bottom.
Perhaps if you intend for post war racing then you might like to add that, also where your pitlane and paddock would be.