BigEndBob,
There's nothing wrong at all with nipping over to another airfield. I do it sometimes, and the terms 'air experience flight' and 'trial lesson' are merely different names for the same thing, namely, Ex. 3 of the PPL syllabus, and therein lies the real issue.
An air experience flight/trial lesson is to allow someone the experience of flying an aeroplane themselves and so they should be allowed to take the controls. They will, of course, experience the other sensations of flying as well.
What should not happen is that a flying school, unless it holds at least an A to A AOC, advertises (note the word carefully) that it's instructors will fly you around the countryside so that you can watch the world float by: that is a Pleasure Flight.
Now, if someone's having an air experience flight/trial lesson and they don't want to take control of the aeroplane you can't (and shouldn't) force them to and so the instructor will then be doing all the flying while they do indeed watch the world float by. The point is, though, you can't advertise this option as a specific product for sale (unless, of course, you have an AOC).
Hope this clears things up a bit.
Regards, GT.