Given that Belgium is flat, going to a ridge site might be a good experience and might also improve your chances of longer flights and maybe wave soaring than if you are only relying on thermals. You could therefore also consider the Midland Gliding Club (the Long Mynd) and Yorkshire Gliding club (Sutton Bank), both of which have good ridges and are not too far to travel from Belgium.
Kent Gliding Club (Challock, near Ashford) would be the nearest ridge to you although its not a very big ridge. Also Derby & Lancs GC (Camphill) has a medium sized ridge.
The Scottish Gliding Centre (Portmoak) also would be OK but it's much further from Belgium!
Lasham is a flat site.
All of these clubs have full time professional operations and offer one week (or more) courses and temporary memberships. I don't think that Kent can offer sleeping accommodation but the others can.
Enjoy!
