Slightly tongue in cheek, Bedfordshire Police HQ has a playing field that the ASU uses from time to time. It is right on the edge of the built up area but approach from the west avoids congested areas. Obviously, permission may be a problem, but you could ask.
It will be suitable for your AS355 Class 1 (although that would be your call!)
Most subtle approach would be to call the Chiltern ASU at Henlow, to ask who to talk to.