As the others have said: 1Gb HD4870 or GTX260 will fit the system and most are under the 150 pound limit.
A few observations though:
Check the cooler design: a great many do not exhaust out of the case resulting in a hot spot under the card which can lead to temperature issues.
Check warranties: a year is OK but would n't you rather have a longer one for a few quid more?
Do not pay more than a few pounds more for an overclocked card: They are only a little faster, so only pay a little more.
Brand is unimportaint.
A few places to look: CCL, Scan, Overclockers, Novatech, Misco, Dabs and Micrdirect.
Here's a nice little deal:http://www.novatech.co.uk/novatech/specpage.html?XFX-48701G