Thumbs up to therapy as being the first port of call. Not a UK specialist, but if I understand it correctly you'd access this initially through your GP. (Don't quote me on that.)
In the meantime, I'd also strongly encourage three other things. First, stick around here; read the posts, make some acquaintances, start to figure out where you fit when it comes to your gender. This site is without a doubt a vitally important tool for bouncing ideas around, getting encouragement, support and taking those first difficult few steps in figuring out who you really are. There's no need to wait until you can see a therapist - you can start now.
Second, you could look for a local trans group and get in contact with them too; attend a meeting, speak to some of the people there. This too will help you figure out where you belong, what you're looking for, and where you want to end up.
Third, are there counselors at your school who you could talk to?
Just don't isolate yourself or hide this away. The NHS can be sloooooow, but there's places you can get immediate support in the meantime.