Not sure I'd bother with the .1 tbh. £500 should get you a half decent set of speakers and amp which will easily fill a room of that size. If you're prepared to go end of line or 2nd hand you should get something that sounds even better. Don't get hung up on getting a sub if you're not happy with the bass, you can always add one at a later date when funds permit. In a 12x12 room i'm pretty confident it won't be necessary.
How about Q Acoustics QTV-2 or their 3.1 QAV System? I'm thinking along similar lines to yourself at the minute though and will probably get a stereo amp and speakers to go with my TV and soon to be blu ray player.