ATI and nVidia are both good brands - best to go with the particular make and model of card that provides the best value - regardling of which it is. See this link about the two card designers:
http://www.upgradevideocards.com/designers.html
Can you provide a link to a UK online site from which you might purchase the cards so we can match prices to performance?
In the meantime, a good place to start a search for good cards is the THG article on the best video cards for the money, updated monthly to reflect new prices and new cards. Note that it includes a very brief description of the performance level of the card - you can read these to quickly see what range best matches your budget and card performance - plus links to reviews with detailed performance benchmarks, most often with comparisons to other cards:
http://www.tomshardware.com/review [...] ,2997.html