I went to City summer '06, they diagnosed me during the session as "Moderately Protonamolous" and gave an informal verbal opinion that it was unlikely I'd be able to pass any lantern test, regardless of where I took it - not surprising as I'd just done the HW lantern and made about 15 mistakes.
You can definitely talk around your chances with them and that's a great reason to go there, but I don't think they would be comfortable giving a formal opinion on whether anyone should pass/fail.
A few days later you receive a fairly comprehensive written report and results of all the tests you've done in the mail.
If you go to Gatwick and fail the HW and Beyne they will definitely put in writing that you are "CP4 colour unsafe" !!!!!!!!
Hope that helps a bit.