Because I know someone who paid a lot of money for a stainless manifold there and despite having the car on site and original manifold to copy didn't do a very good job. He's been back twice since trying to sort it and it still doesn't fit correctly.
I got two from London stainless. First car got a centre system and tailpipe (empty silencer and twin pipes).
Second system: H120/GLS manifold and 2 inch pipe all the way to the back with DTM style tail pipes (and no baffles in the back box). Not cheap but very good quality and fitted perfectly and sound very nice