Jim Warren gurgled happily, sounding much
like they were saying:

Also, fitting a tyre of a different
specification can fall foul of construction and use regs as the car was
type-approved for the UK market with a certain spec. Badger.

Does this mean that everybody who has replaced the steel wheels and 80
profile tyres with alloys and 70 profile tyres could fall foul of a
Jobsworth quoting the Construction and Use regs?

No, not necessarily. He said "can", not "will".

OTOH, if you take high-spec tyres off a quick, heavy car and put
something that's absolutely not up to the job on, then - yes, probably.

Speed and load ratings are likely to be the key.