RCA out is only stereo.
Only analogue speakers are available at that price. They are only of real use connected to a PC though, as TV's, Sky box, consoles etc, only have digital connection for 5.1 speakers.
Depending on the TV, a lot of cheaper TV's only output stereo via an optical connection as well, so any Sky box would need to be connected directly to the speakers, via optical. AFAIK, Sky doesn't do 5.1 via HDMI.
5.1 speakers with digital connection will cost you somewhere in the region of £140+, for a cheap all in one DVD based system. A lot of the cheaper ones have limited connections. Some have no digital input at all for external devices, so you'd have to be careful if you did choose to get something like that. They are not that great either.
For a decent budget 5.1 speaker system, with decent amount of connections, you're looking at £300 at least, unless you can get a bargain 2nd hand.