There's a variety around, though as per normal, depends upon £.
Cyrus use shoebox style cases. No idea if that's small enough.
Musical Fidelity did some rather cute cylinder shaped amps. Never heard them.
The other option is a decent digital unit, which will also have the benefit of probably running cooler, which may be a benefit if you're going to put it in an enclosed space. For a cheapy system, might be worth looking at some of the tripath based units. I'd avoid most of the chinese stuff, but there's some groups in the UK who sell them at sensible prices.