Its pretty damn hard to find a decent 5.1 sound system for £100 pound. You could get a darn good 2.1 system for that price.
You best bet is to have a look at the acoustic solutions surround systems. They are cheap and good quality. My brother bought one (5.1) a while back out of argos for about £100 out of argos and it sounds pretty good and it has an optical input.
If your using it with your pc i'd advise on getting a creative x-fi sound card. You can get an oem one off of ebay for about £30-£40. You'll notice the difference in sound quality compared to the crappy integrated sound.
Still haven't got a system yet wanted a ps3 first : multi :. Decided i want the amplifier to have multiple inputs, one for the ps3 and one for the PC and maybe 1 more just in case. Preferably with the optical still for ps3 and aux for PC. I would maybe get a separate amp and speakers if necessary and it doesn't have to be a 5.1 a 2.1 would do.