I agree that, like most things, it helps if you're lucky enough to get a personal recommendation from someone you trust.
Failing that, I would make the following observations.
1. It's not quite like looking for a tutor - chartered educational psychologists are regulated, with accreditation through the BPS, and there is now a requirement that they are registered with the Health Professions Council.
2. They are highly qualified.
3. No matter who you go to, they are all likely to come up with the same sort of report, using the well-respected WISC or BAS tests. I suppose some EPs might appear 'nicer' than others, but I'm not sure the outcome will be any different!
4. If you decide to pursue this option, I would suggest ringing up two or three local EPs from the register to discuss why you're thinking of having your son tested. I doubt that whether they are any 'good' is likely to be an issue - you might wish to select on the basis of whether you get the right 'vibes', how experienced they are in providing reports for the sort of purpose you have in mind, and what their fee will be.