Read. Read some more. Then read more.
It's great that you treat Maths problems as fun puzzles, because that's what they essentially are. Don't worry about those challenge papers (UKMT etc); they aren't really an indicator of much. I got Silver in Intermediate and Bronze in Senior; one of my friends got a Gold in Intermediate and Gold in Senior and then went on to get a U in three A2 Maths modules.
Back to reading. Basically, that's all there is to it. Don't go on the computer all the time (I'm not saying you do, but it's a problem I have, and so I banned myself from going on it on weekdays. I read instead now), and use your free time reading. I would highly recommend Jeff Forshaw's and Brain Cox's books (Why does E=mc
2 and The Quantum Universe) as they are very non-mathematical, and explain things like Special Relativity and basic Quantum Physics very well. Also, Chad Orzel's books are very good, and highly amusing (to me, at least). Finally, Feynman's QED is very good. All those books are aimed at the casual reader, so if you're interested in the subject, you should be able to read them. Don't expect to understand everything the first time, you'll 'get' more and more of it as you go through GCSE and A Level.
Also, try to keep up to date with the latest Physics developments. It will help to keep your interest up. For example, do a bit of reading around on the Higgs Boson and the Higgs Field. It's an extremely interesting area
.
On a final note, GCSE is very boring. A Level isn't much better. Don't let that put you off
. Read, read, read and then read some more.