I'm just wondering why you can't land at Wethersfield. I've been into MOD sites (Coningsby & Coltishall) before, the only special requirement being insurance. Either Crown Indemnity if your insurer will arrange it, or a one off daily waiver which was about £8 IIRC. Admittedly this was a few years ago so if the current security situation has changed things then I apologise. I notice that the 614 VGS Gliding School are based there, have you spoken to them?
Regards.