That's a fairly large room to fill, and what will be needed to fill that room will depend on the volume levels you require. The bigger the sub the better, so something like the SVS PB12 NSD will do the trick. You'd be looking at receivers around the £1k mark, so check out the Onkyo 818, which will make the most of almost any speaker package with Audyssey's XT32 room EQ, currently one of the best available. If you can catch an outgoing model like the Pioneer SCLX75, Yamaha RXA2010 or equivalent, you'll get more out of whichever speaker package you choose. For speakers, try Monitor Audio's Bronze BX2's. If you can house it, try and use a BX2 as a centre too, which will help keep your soundfield more consistent, and will outperform the matching Bronze centre.