+1 for this question as well. Can be Slough area. There is a powerflow branch here but im nto sure how good are they. There are many bad opinions about powerflow lately.
[EDIT]
Actually i know very, very good custom exhaust specialists that do exhausts for race cars as well. They are called Simpsons exhaust. I've been there - garage looked mint and tidy. Some beautiful cars around as well. Only
problem was with quote for a turbo back exhaust for my cupra. 2,5 inhc TBE with sports car - £1000. I could never justify that on my £2k car

. But top work by those guys from what i have seen.
http://simpsonraceexhausts.com/