Hey clipper, sorry i can't help, but for what it's worth, i'm in auckland and we recently had a cold snap - my turtle went nuts!!! went back and forth between not eating & tucking up in his shell on the floor underwater, and freaking out, careering around missing his turns so he was smacking into the glass.
Was pretty scary, but my point is that they're clever and sensitive, so if you've had a weather change, or something new in the environment etc that we normally wouldn't think of, it could still be bothering your turt.
However, sounds like an irritation of the throat. Best of luck with getting him looked at by an experienced vet - ones that have worked in the UK have usually seen lots of turtles