Had a DT175 frame straighened years ago, fairly uneventful.
I take it they mentioned that the front end will need stripping off.
It would be worth mentioning to them that the rear wheel may have the wrong offset. I suspect they will use the swinging arm pivot as a reference for the headstock, but it may confuse them if they check the wheel alignment after straighetening it and find the wheels dont line up.
If your convinced its bent I'd get it done before Pembrey, especially if scrutineering pick up on it and stop you riding.