Fair enough, I missed the word 'legally'. The trouble is that under the S.165 Road Traffic Act, if the car was ever stopped and found to be illegally modified, there's the possibility of immediate seizure as the insurance would be invalidated? (Unless the insurance company was happy to still provide cover with an illegally modified car, which I find difficult to believe).
It's the old adage, "Don't break the law when you're breaking the law". Driving around with no Cat is extraordinarily unlikely to attract any attention, but driving around with a straight-through silencer is exactly the kind of thing that does.
Under Section 59 of the Police Reform Act 2002, officers can seize vehicles driven in a way that causes "alarm, distress or annoyance" (such as excessive exhaust noise or reckless driving). I'm sure Montegue would never drive recklessly on public roads, but the noise alone might be a concern?
I'm sure Monty is capable of making his own decisions, but for a car used regularly on the road, sacrificing a few bhp to remain legal would be my preference.
Tim H. 1986 4/4 VVTi Sport, 2002 LR Defender, 2022 Mini Cooper SE