For surround sound, you need something at least with 5.1 surround sound, Dolby Digital (or DTS) decoding and an SPDIF, or Digital Optical input.
That will let you plug an optical audio lead (I love the way that sounds like a contradiction) from the PS3 straight to the Surround Sound.
However, if you're not spending much on a surround sound system, you wont get a very good quality system. I'd recommend waiting until you have a £150 or so saved up for a dedicated decoder system (or AV receiver). It won't be brilliant for that price, but it won't be tinny or crunchy (to untrained ears) either.
Just remember, you need to go for a decoder and amp for surround sound, not just a set of speakers. The TV will only output through a simple stereo (two speaker/headphone) jack, or through a Dolby Digital stream over either coax or optical (most likely optical). Speakers don't know what to do with a Dolby digital stream, so you need a decoder/amp to do that bit, which then sends the correct sounds to the correct speakers. Quite often you'll see these listed as Home Cinema Systems - rather than the real terms
The problem here is that most are just DVD players with additional speakers on them. They will play their own DVD's in surround sound, but don't have the inputs for your TV or PS3 to play through them. Places like Amazon don't have enough details on the product descriptions to help you out here.
You're getting into complex territory here now. If you're already feeling lost - then you need to go and talk to people. Either sign up to the AV forums and look round there for advice, or go to the local audio shops and get an idea of what there is out there you can buy. The problem here is that places like Currys and Comet will fleece you - and often the staff don't know a great deal either.
I've just had a quick look at a few places, and I can't find anything I could recommend to you Ollie. They are either too expensive, or I can't find enough details to be able to recommend them to you.
Just remember:
1. Digital Optical Input/SPDIF In
2.
Total RMS is a con. If it's 15Watt Total RMS (which in itself is a wrong term) - that means each speaker will output about 3Watts - which is nothing and will crackle and spit at loud volume. I wouldn't go for anything less than 10 or 15 watt PER SPEAKER in a bedroom situation (for you who is used to just a TV speaker).
Having a PS3 hooked up to a pretty cheap Dolby Surround amp myself, it's brilliant. Games really come alive when you can hear cars coming from behind, or a shot over your left shoulder. Likewise, films should always be listened to in surround sound
Sorry I can't really help a great deal more - it's a very complicated area for the uninitiated...