DELL UK (and all of the EU) offers the 6950 inside the XPS 8300 so unless they have a different PSU, it's officially supported. Perhaps try to find out what PSU is in the XPS 8300 in the UK.
http://search.dell.co.uk/1/1/141761-...hics-card.html
I think Dell USA/Dell Canada just don't want you to be able to add a 6950 into the XPS because they want you to spend more money and buy an Alienware system for that.