Most military airfields always have been open for visiting GA, Subject to PPR and operational requirements. I've personally been into Coltishall, Coningsby & Wattisham in a microlight. Granted these were all a few years ago, and security has probably been tightened since. You'll have to produce evidence of third party insurance (it used to be £7.5m) or pay a one-off daily rate to cover it instead. Bear in mind that the military do things their own way, and from my experience, aren't always familiar with small circuit patterns, and light aircraft being able to land and take off in short distances. Also beware of jet exhaust - I wasn't advised of a Jaguar running on the apron, and had a scary moment while taxiing past it...