peter - a FAA instructor in the south of uk told me that. Without dft permission they (the instructor) considered even a positioning flight with a student to be aerial work . I wanted a few hours and a private check ride (so I can get rid of my 61.75) They apparently position to Le Touquet, you get your self there, and subsequently expect the examiner to attend there. They considered that it was an issue to get dft permission, although several posters here indicate otherwise, that may be because they don't actually own the N reg they were intending to teach in.
I did consider it to be a conservative response to say nothing of the cost and effort involved and that was a factor in deciding not to take that option but to go to the US instead. Regretably no checkride in the end due weather, but going to do it next time I am over there instead, hopefully followed by a chunk of IR training.
The other issue I found with some FAA training in UK is that some places quote only full day rates for instructors , not hourly. Not sure that works for me.