Just to give you a little info on Oxford, where I did the modular course. I definitely give Oxford the thumbs up. I know that the customer service is not the best in the world (experience had), but in my opinion if your sole aim is to give yourself a good chance of getting in and out as quickly as possible with a fighting chance of getting through the exams first time then you will not be disappointed with Oxford.
I'm sure that some of the other schools are equally as good but if you want to hear it from the horse's mouth then there you go. If I had to go and do the ground school again then I'd be straight back to Oxford!
Hope that helps,
Barnaby