I cant comment on the quality of these guys work. But Ive bought some Magnaflow stuff off them before. And they're at Heathrow

http://www.profusioncustoms.com/pc/contact_us.php

Goodfabs isnt too far away. Although you'd probably need to book well in advance and I'm sure they'll be more expensive than your average fabricator.

http://www.goodfabs.com/

Or also near Heathrow and top quality

http://www.simpsonraceexhausts.com/page11.html