Depending on who you talk to there's many reasons for the lack of top of the range video cards...
No DDR3 memory at 1.6ns
Low yields on the high speed GPU's
Bigger die size means less chips per sheet of silicon.
And so it goes on... This is a worldwide problem, not just UK.
I have found Overclockers (click here) have good supplies of GeForce 6800GT cards. I recently got a Gainward 2400 Golden Sample (factory overclocked to 6800Ultra speeds).
A friend of mine has had a Radeon X800XT PE on order with Dabs for about 3 months now.
I have recently read that there are some stocks of X800 and 6800's trickling through to retail, so fingers crossed.
By the way, Overclockers UK have the cheapest 6800GT I have seen (about 240inc VAT), and they also normally have cards by the 'big' makes such as XFX and Gainward for very reasonable prices.