I used a place called "Cats Eyes Driving School" which is based in South Wales. I live in Harrow, but decided to go out to Wales for the test because there's almost a 6-month wait to take it in the London area, which would have meant putting "L" plates on the car and no motorway driving for me after next week (when my courtesy year on my US license expires). I really didn't want that so I signed up for a test centre with a relatively short wait (Barry--about 6 weeks). The school booked the test and they arranged a very nice B&B for me, also. The whole little trip was very pleasant except for the driving lessons and test.
I was advised to take the lessons where I was going to be taking the test. I cannot stress how important this can be if you are taking your test in an area which is unfamiliar to you. Doing well on the test is much easier if you know the roads you'll be using.
The whole adventure cost us £400 but now that I have that pass certificate in my hand, it seems worth it. My confidence flagged, too, as we were finishing up instruction on the day of the test--my teacher was very picky and didn't let up on the criticism as the test got closer. This made me a bit panicky so I told him to ease off on the perfectionism a little, because it was making me think I wasn't up to test standard. He complied and after that things got much better. I generally found him to be an excellent instructor, if a little tough.
However, caveat emptor, because I am very used to men being tough on me--been dealing with orchestra conductors for many years and some of them are "right bastards" indeed!