Milltek non-resonanted vehicle inspection don't like

[bam_bam]

As the police like to tell me every time I get pulled over, in the UK it is illegal to modify the exhaust in any way that makes it noisier than standard.

You haven't modified the exhaust, you've replaced it. There is no actual MOT noise limit for an exhaust, it'd be down the discretion of the tester. Find a petrolhead MOT guy and you're fine. There are other parts of the test that can cause a fail, like, you are 'spose to have the appropriate number of silencers for your make/model. Out on the road it's a little different, you can still refer to the valid MOT you're rockin' but plob plod can cite you for excessive noise, disruption of public etc.