Your best bet is to buy from one of the American retailers. From previous experience you can generally guess the UK price for mac upgrade cards by removing the $ and replacing it with a £. It's far cheaper get stuff fedex'd from the US. I can recommend OWC and they do have the the X800 list for just shy of $500. With VAT and shipping expect that to mean £340 delivered.
I have to concur with caveman, I use OWC alot and highly recomend them its just so much cheaper now with the great exchange rate, I'm not sure how you stand with import tax on a vid card though?
In my experience UK customs are strangley selective on how they calculate and charge 😕
My understanding is that Fedex shipments ALWAYS have VAT charged on them. But having said that my last shipment hasn't been charged yet....
I recall a chat with a guy at Parcelforce one day who said that for USPS shipments that arrive via Parcelforce the customs people choose particular containers to check when they arrive. If your stuff is in one of those containers then, bad luck, you're paying tax. If it's in the containers they don't check then you get away with it.
The big plus with getting Fedex from OWC is that your stuff will be at your house in two days from shipping and you can track it.