Cheap is a relevant term... how much you looking to spend?
I would recommend the Onkyo HTx 22HD, but this may be out of your price range.
Maybe try: Sony SRSD511.CEK
They have two inputs so if you were using them with pc as well, you would not have to swap cables around.
Dunno if you will get them for £50, but might be close.
Edit: Oh yeah, defo nothing with optical around that price.
Edit: Never mind that suggestion, I just remembered you wanted 2.1 setup.... while there is a 2.1 in the same range, i don't think it has multiple in's.

2 input (MIX) to connect to PC and other devices (WM)
What are MIX and WM inputs?
logitech x-230 absolutely brilliant for the money, i've used them for years.
Nicely priced aswell.
What connections do they have on the back?
i just connect mine straight in to my TV (which in turn has the sound from the 360 running to it) they're built/wired with games consoles in mind though and connect via a standard 3.5" stereo jack which you can buy an adaptor for to allow for RCA connections (which is what i used to do).