I'd think that for £6k you'd be pushing it to find a decent glass outfit. Having said that, the Bristol University GC Astir is for sale asking £6.5k (but no chute or logger) - the adverts on gliderpiot.net are a good place to start looking, though there's not much on offer at the moment.
If you can up your budget a little or cut a good deal, then your choice is probably Astir, Libelle or ASW15.
Of those three, the Astir is by far the biggest cockpit - I'm 6' 1" and flew one for some years, and it would have taken a much bigger pilot very comfortably. Have only sat in an ASW15, and it was very tight indeed on me. Libelles depend mainly on breadth of shoulder and length of body - I can fit comfortably, though have yet to fly one.
All three are good gliders. The Astir is probably the easiest to fly by all accounts, and the most comfortable for big pilots. Its weaknesses are a cast aluminium frame, which can crack through heavy landings, and a rigging system which has a reputation for being awkward (but is fine once you know what you are doing - I can consistently rig one with a single helper in 10 mins). Built like a tank otherwise.
ASW15 is much lighter and reputedly more responsive. Offset tow hook is known to require more careful attention when launching, otherwise no issues.
Libelle is reputed to be the nicest handling but hard to fly well, requiring good handling skills. Airbrakes much weaker than the other two, so needing good speed control on approach.
For half the money or less you might find a good Pirat - big cockpit, good airbrakes, quite nice to fly. However, rig and derig it a couple of times to see if the heavy centre section is a problem - it's really a three-person rig. If I was looking to get lots of cheap flying and go for my 50k if a decent day turned up, this would be my choice I think. If you want a glider which will take you into regular XC, then I'd go for glass. The other thing to remember is that most wood is very hard to sell these days, so you might have to write off the cost of a Pirat if you trade up, or just syndicate it out to local soarers.
You'd also fit in a K6E I'd guess, and that's a very nice aircraft which should be under £6k, but you need to be a good pilot to get one round a 300k.
Finally, have you thought of asking at your club if anyone wants to offer an insurance share? A year or two's experience would give you a much clearer idea what you want to buy.