I was pulled a few years ago in my mk4, had wide rears and a low offset that probably hung out the arches a good 15mm.
Got pulled and they said its illegal, advised them it wasn't as the tread was covered by the arch. The reply was they'd happily ask vosa to come down and check the car thoroughly, and the fact there was some rubber on the arch from where it caught the bodywork wasn't winning me any battles, so I changed my wheels.
The cops admitted it was a grey area. Probably because they didn't know the law fully. And as I work for the police, didn't want anything coming back on me. Iv since looked it up and it is not to protrude 30mm past the arch and the tread is to be covered. So "technically" it's ok.
Hope this helps