Even if the field doesn't have radar it's worth squawking 7600, as someone will probably pick you up; e.g. if you're in the hold at Oxford and lose radio communications, and squawk 7600, little alarm bells will go off at Brize who can then ring Oxford and say 'you've got a chap in the hold squawking 7600' so they can alert other traffic to get out of the way sharpish...
Tim