There is a lot of discussion on various forums. Be a bit careful taking advice since they do change and people's experience may be well out of date. General consensus seems to be that Eastnor, Skipton, Peckforton are the best, but they all have the basic facilities. Some have more off road space than others. They do tend to get booked up - I'm booked at Eastnor in April. It's a weekday but even then I had to book about 4 months in advance. Apparently Eastnor has only one DS available. The wait may be shorter with other models. The usual programme is a drive on road, followed by an obstacle course with instructor driving, followed by the same with you driving, followed by some off-road. Apparently some centres will allow you to modify this - after all if you drive the car on road all day the first part of the programme would be pointless. On a personal basis, I had a poor experience at Luton, but that was a few years ago. Even so I don't recommend it.
Best experience I have had to date was in N Wales, when a member of the FL2 forum organised a private full day using our own vehicles. never did get all the mud off...