I cannot find any UK sources that I would trust myself for a BTX powersupply. However I will make a few comments along the way and in the process bump this thread for tomorrow morning which should get you an answer.
I would be doubting that your P4 system will benefit from the use of such a powerful graphics card and that your P4 will just lag behind and therefore make very little difference to your overall performance benchmarks. This would also be a bit pointless as most modern games (at which this card is aimed) need a processor with a lot more kick.
Also doing some searching on ebay they even made a few celeron boxes of these 5150s and these would be even worse than a P4 in terms of kick. If you really want to use that graphics card, you may want to consider selling your current machine and moving into the dual core level of computing, as I dont think you will see all that much of a difference in performance. Others may argue to the contrary but I think the processor will act as a very big bottleneck for your performance and it will be like adding a Ferrai engine to a Ford Focus. Too much power for it to handle propperly.