(I can only pass on what I know about math resources.) Someone at a homeschooling website recommended the
Mathematics Enhancement Programme. I checked it out and it seems good for enrichment/practice. For actual worksheets and problems, you might like the Singapore Math workbooks and Challenging Word Problems / Intensive Practice. The workbooks aren't as dreadful as they may sound-- they actually seem to shake it up quite a bit with different sorts of games and puzzles (I didn't realize this until just recently when someone recommended them, as I'd never cracked them open since DS seems not to do as well with a lot of drill).
IXL and ALEKS might suit your son well too, for online resources; IXL is just for skills practice but is quite well suited to a kid who likes to skip around and doesn't need practice in everything. And, of course, EPGY might be an option; I'm not aware of any restriction on UK people if you sign up through the open enrollment option.
ETA: yvonnemommy wrote today in
another thread, "DD has been using math pro on beestar. It offers online math worksheets. You can have your kid try the Gifted Talented Math."
