Holy thread revival! I don't know if there's more than one BTB exhaust company, but I called the one in Daventry and they said I'd be looking at a couple of grand for a custom exhaust from them for my 172 Cup.
Powerflow Mitcham have quoted me £600-650 over the phone for a full cat back system with two silencers and I'm going to Powerflow Bromley (STS Tyre Pros) next week to get a quote from them, so will report back.
I've currently got a full PMS silenced stealth system with sports cat that's probably done <500 miles but it's just too loud for me, I want something a lot quieter.