The most discreet thing is going to be a soundbar, but you'll be limited for rear effect. Wireless rear speakers are going to have to be active, meaning they're not going to be small, and even if they're run by an amp you can tuck away behind the sofa, theres going to be wires involved. The simplest wiring will be the traditional way - from an AV receiver at the front supplying two speakers at the back with speaker cables.
Other than in-wall, there's no real high quality compact speakers. The smallest that spring to mind are Bose and Boston Acoustics Soundware. You could look at KEF's on-wall T Series which are very flat, and not obtrusive as far as room space is concerned. Going a little larger will open up your choice vastly. B&W MT50 or MT60D, KEF KHT 3005se, Q Acoustics Q7000 and Monitor Audio Radius HD models will be well worth looking at. Mix them with a good £500/600 AV receiver and you'll have a great system that you won't regret buying, and even if they are slightly bigger than you'd have liked, the quality will assure you that you've done the right thing.