Funny how some threads come back to life long after the OP's trip should have taken place!

But in case anyone else happens to start their UK trip by flying into Mildenhall, as the OP's parents were, odds-on that's because they're USAF. As Mildenhall is out in the middle of nowhere in the Suffolk countryside, I would have thought that USAF locals would be a good place to start asking for car rental options.

The Scots might have something to say about the title "UK->Scotland car rental" - but then again they might not.