A lot of UK gliding clubs now have sponsorship schemes for under-18's = free training. Check with your local club. It's been said that all glider pilots go on to make good power pilots (before someone jumps in, it may be an exaggeration, there's always an exception somewhere!)
And good on yer for having some ambition at 13!
Just as a passing comment, there's one of the guys from our gliding club going through his CPL training. They've been doing their ground school for 6 months and are only now going to the USA to learn to fly! Apparently most of the people on his course have never flown yet. I find that positively scary, so do me a favour, follow your plans and learn to fly
something first!
p.s. the younger you start, the quicker you learn