pafanador,
Check with the BGA before you go that route.
Can you get an actual EASA SPL in the UK yet? I was under the impression that wasn't going to happen until 2018. Unless things have changed, you get a BGA gliding license, not an EASA SPL.
There are gliders and, I presume, clubs and examiners in Spain. In Spain you can be sure of getting an EASA license and don't have to travel. Also, if you're going to join a club so you can soar in Spain, I would encourage you to do the work there.
If you do go to the UK, there are three clubs I know of that function in a quasi-commercial manner and are quite open to people coming for a course or a weekend as a day/reciprocal member. Look at Booker, Lasham, or London Gliding Club. They are all within an hour of London. I've enjoyed flying at all three - ridge and thermal at LGC, "just" aerobatics at the others.
Terry