Heady, Hi.
First, there is no official (CAA-issued, internationally recognised) UK glider pilot licence. We fly gliders without needing one in this country, any more than you need a yacht master’s certificate to take out a rowing or sailing boat.
Second, experience gained elsewhere will stand you in good stead in the UK, subject to check rides and refresher training if you need it. If you get as far as having a licence abroad, I would expect it to be convertible to a UK licence when we have one.
There will be a UK licence when EASA FCL comes into play, 2012-2015 (as far as one can tell at present). People who already have sufficient gliding experience and qualifications by 2012 will have some sort of “grandfather” right and be granted a licence during the transition period.
The right experience is probably (IMHO – not seen anything official) to be at least Bronze, more likely Bronze plus cross-country endorsement, and possibly after at least doing one solo cross country flight of say 50km. It might need as much as Silver – all the above, plus gain of height of 1000m and duration flight of 5 hours – but I doubt it, as that would be more that a PPL has to do for a power licence.
Look at the BGA website if you want more info on these things (Bronze, Silver etc. are peculiar to gliding).
(Notwithstanding my first paragraph above, don’t get confused if you read about the BGA Licence. There is one, but it is not CAA-issued, nor internationally recognised – it was created in the hope of the latter, but has not worked in France for example, as I understand it. We have to get a temporary licence to fly in several countries abroad. That will no longer be needed when we all have EASA licences – probably the only benefit of this whole sorry EASA bureaucracy and added cost without added safety.)
Chris N