http://www.legislati.../1986/1078/made
Boom, there you go. Reg 63 covers wings (referring to wheel arches, not spoilers) and states:
OR
http://www.legislati.../52/section/40A
The RTA states:
So sticky out wheels and tyres could be proven to be hazardous to pedestrians in court, but again there's NO exact numbers or figures for this. You may well read about a 30mm protrusion limit for wheels and 75% of tyre covered by the plan view of the bodywork, however this is for SVA (now IVA) cars only and DOES NOT apply to modified factory-built cars. It all depends on how a court views these laws, rather than defined numbers.
And that, ladies and gentlemen, is the official answer. No ifs or buts, that's the exact law which you will be tried upon in court.