Mate il be honest with you now,Theres no need to spend £100 on speakers
You wouldn't be saying that if you had the Z-5500s.
For the budget, I would definitely go for the G-51s though.
It isn't true 5.1 but if the speakers have a decoder built in that supports dolby prologic then you will get 5.1 that is quite accurate considering it is from a stereo source. I play Halo 3 on my 360 with my speakers set to Dolby Prologic mode and all the sounds are directional and it sounds just like surround sound. I know the Z5500s have them, not sure about the others, I think the "matrix" mode might be what some of them have.
X-540 or G51, although these also look nice:
http://www.overclockers.co.uk/showproduct.php?prodid=SP-118-CL&groupid=702&catid=22&subcat=165
Well probably not the best for a £100 budget but i must reccomend the Logitech X-540s
http://www.ebuyer.com/product/116853
£50.24
or for your full budget
http://www.ebuyer.com/product/133295
£100
Either way with Logitech your going to get quality.