£6K is a very, very tight budget unless you are considering wood as well. You will have to look hard and both being big-ish lads doesn't help.
Have you looked at 'gliders for sale' on the Gliderpilot.net website?
Glider Pilot Network > Classified Adverts
Also, some (maybe most or even all) clubs don't let you fly a privately owned glider until you have a Bronze. Better check at your club - it would be a shame to have one and not be able to fly it.