If you have a friendly MOT tester then a decat downpipe is worth it in terms of performance and cheaper than a sportscat system. You'll need a remap if you do either decat or sportscat if the car is current in standard map or mapped for the oem downpipe.
The Prodrive downpipes are good but getting hard to find as you know. Otherwise if you want it MOT friendly just look for a sportscat downpipe, but they're not cheap (£400+)
http://scoobyworld.co.uk/catalog/pro...oducts_id=3396
https://www.haywardandscott.com/suba...t-new-age.html