I don't think I've seen Booker mentioned yet. I've flown at Booker, Lasham, and Dunstable. Enjoyed them all.
On the continent there's Wasserkuppe and DASSU (Deutsche Alpen Segelflug Schule Unterwössen) to consider. I've been to Wasserkuppe a couple times; want to do some Alpine training at DASSU.
Wasserkuppe is easily drivable from Belgium. They have no issues doing training there in English.
PS If you are ever near the Wasserkuppe, visit the museum at the home of soaring. Wasserkuppe celebrated 100 years of soaring in 2012.