I'm sure there are exhaust specialists in the UK, but here in the Netherlands there is just one company that can do anything you want for a reasonable price: Exhaust Power Supply (or EPS)
This is what they did to/with my car:
https://www.youtube.com/watch?v=Dd4HcJwt4Cs
The sound of my car is just awesome (I think) and the price was very okay in my opinion: € 1.250,00 (all custom made in SS in a day).
Perhaps it's an idea to visit the Netherlands with your car, make an appointment with EPS, have your custom made SS exhaust fitted, and drive home with a big smile on your face...
http://www.uitlaten.com/main.html
When you visit them they can try several set-ups (one or two mufflers, what kind of end-tip(s), what diameter of the tubes, et cetera): it all has an impact not only on the sound, but also on the 'power'... After they fitted the exhaust to my car, the car not only sounded much better, the engine stayed cooler and the car gained 4 bhp...