You're in luck, in that the two roles you're interested in there don't really require specialist *game industry specific* training.
To be a concept artist you basically need to be a
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traditional artist. You don't even necessarily need to be able to do art on a computer (although it helps).
The most important thing for a concept artist is the ability to *quickly* be able to render into art someone elses ideas. Not all concept art is of the print quality standard you see in artbooks.
Knowlegde of how 3D game art and animation works is useful, as you can factor feasability into your concepts, but none of that is as important as being able to do
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quality traditional art quickly.
In terms of writing the plot, that's usually a freelance position these days. And there's no specific way of becoming a game writer as opposed to writing for film, comics or TV. Learn to screenwrite, and you'll be able to do writing for the games industry. But you'll also be able to write for all sorts of other industries too (and you'll need to, as I said, most writing work is part time contractual rather than full time).
As you haven't even reached your GCSE choices yet you still have time to experiment with both. In the UK you can't avoid doing English, but you should probably take Art as well. If your school offers technical drawing (one of the CDT disciplines?) then that might be worth a go too. But you're some way off needing to think too much about specialising, that's what your A-Levels will be for. And by then you should be a little clearer about whether you're developing into a writer or artist (or both, if you're
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).
The last thing I'd say, and this applies to pretty much any discipline in the industry, is that there's no substitute for just doing the thing. Join a fanart community and start getting comments on your art that way. Write short stories (we have a palce on the forum for fan fiction, other places do too). Practice practice practice.