Hi there force, welcome to the board. I guess you're in the UK... Somewhere "up north" if i can detect your "accent" correctly? I'm down south in London.
I'm not really sure what advise to give you. If you cherish the relationship that you have with your mum, you can learn a little bit more about the JW's from here and be VERY prepared to ask her certain questions and lead her down logically irrefutable lines of reasoning.
If you're not confident in your ability to do that, under no circumstances must you go to the Kingdom Hall again. Even if you have to go to a "Memorial" once a year, try to keep contact with her JW friends to a minimum and don't engage in any religious discussion with them. Be firm that you don't like talking about religious issues, or that you really are not interested. Never feel guilty about doing this, it's your right. However, as AlmostAtheist said: play it cool... don't let it all enrage you.
As for the TV, Internet and cinema problem*, keep close to the friends that you already have, become even closer to them and try and do things around their houses as much as you can. She will most likely try to cut off your contact with them. You must fight to your last breath to keep your friends... if you don't and you lose them, it will be very difficult for you as you are very unlikely to find true friends in the JWs.
I don't know what you are planning, but keep your eye on what you want to accomplish in your life, whether it be a career or further education. Put you're whole strength into your GCSEs and don't let anything distract you from those, they are so important. Be selfish, but try not to deliberately start conflict with your mother. Don't use terms like "it's not fair" or "i don't care" with her, but reason with her as much as you can
Things will not likely get easier for you as long as your mother remains in the religion, but as long as you can keep your mind free of their brainwashing tactics, you should be ok. Two years is a long time to wait when you're 15. I hate to say this to you but... lie to her about which film you are seeing, and be a good lyer, lol... a 15 year old has to have fun.
* p.s. As for Little Britain, you have Bubble De Vere who is nude and grotesque, Ting Tong the Thai catalogue bride, Daffyd the only gay in the village, rubbish transvetite Emily Howard etc, all of whom exibit qualities that the JWs think are immoral or satanic. In the same way, Lord of the Rings, Harry Potter and The Chronicles of Narnia are forbidden films. Not because anyone specifically says that they are bad, but because every JW should "know" that they are.