Why do you need a trail centre. Buy an OS map. Seek and yea shall find…
Bedgebury is OK, but I can think of tons of far better natural trails in the area… and despite being pretty densely populated, very few of the South East's denizens actually venture off road on bikes, so once you get away from the honeypots around Surrey, you can quite often find yourself riding all day and not meeting another soul. When I ride in the Lakes and Wales, I often find it too busy! I live in Kent – am about 3 miles from the M20 so don't live in a particularly remote area, but can do a 30-40K ride from the house – probably 80% offroad (lots of woody singletrack), and maybe see a couple of walkers or horse riders all day.
I live just down the road from East Grinstead (Crawley Down) and do most of my riding in the area, Tilgate forest, Holmbury/pitch/leith, south downs (now a national park), Bedgebury, St. leonards forest, Friston forest all within an hour, could be a lot worse imo.
Life is full of compromises. Perhaps you could move house if the riding available is unsatisfactory. However to do so would more than likely diminish your pleasure in another aspect of your lifestyle.
Like someone above said – buy a map, it's all there. I like just riding the countryside, but I like trail centres too. I would def join in trail repair days, no problem, just ask, it would be a great workout.
About resting trails during wet periods. When horse riders rest trails during wet periods I will do likewise, until that day…