You'll most likely need 6-3-1 "extractor" design unless you're racing the car, and it's a good idea to get stainless steel for longevity. And, a stock exhaust system may negate most breathing benefits gained by fitting headers, so get a good system to go with your header.
The British pound exchange rate is fairly good now, if shipping doesn't kill the deal. Canley is a good bet, maybe Rimmer Bros.
SpitBits has headers, too, but I haven't checked recently. I'd check with Nigel at SpitBits first.
Let us know . . .