hate to sound like a stuck record but if you want someone for <£10k with similar performance to a focus ST but a better more comfortable interior you've just described an e46 330i or Ci pretty much perfectly
This. Didn't want to say it and have every man and his dog claiming that "OMG THIS FORUM ALWAYS SAYZ DAT!!!!111".
Why do you want to change anyway? Assuming you don't want a 330, i can't really think of something that'll be a step up from your Focus ST. Something like a Golf GTI will be much the same as your Focus.
Can you get Z4's for this price? I know 350z's are starting to come down, but you won't find a good one for 8-9k i shouldn't think.
If you don't want a 330, i'd probably just stick to with your ST or if you fancied a change for the hell of it, maybe a Golf GTI (although it would be pointless).