Out of curiosity, I guess most people have never tried...but would you have much time to get out and activate a parachute in a mid-air collision?
I know of an incident where a pilot bailed at 800ft and survived without injury, but I'm told most are rated at 400-500ft minimum opening height.
Since Mary has started the club-plugging, I'll highly recommend
Kent Gliding Club at Challock (near Ashford in Kent) which has two railway stations nearby and plenty of members willing to give lifts.
It's relatively small compared to the likes of Lasham but has an excellent atmosphere with 7 days of flying in the Summer. PM if you want more information or want to to arrange a visit.