I had a full exhaust fitted (behind the downpipe) last year by Cybox in Bromborough on the Wirral:
https://www.cybox.co.uk/custom-build-exhausts/
They built a custom 2.5" exhaust with two boxes and a Magnaflow cat for £400. They originally quoted £500 but decided the down pipe was good and didn't need replacing so reduced the price accordingly.
I dropped the car off at 8am and it was ready at about 3pm.
I would happily reccomend them as they did a good job and i thought the price was very reasonable.