its not illegal for the tyres to stick out, only the tread. Obviously you'll attract more police attention possibly. Although i've got 18x9.5 et47 and 20mm spacers/adapters giving a final et of 27 on the rear, a 225/40/18 tyre, the treads under the arch the rim is about 15mm out from the edge. Maximum legally you are allowed is 30mm but this is a very grey area and most coppers will not not know either, so you could well get a ticket.
I've been running mine on my golf then the avant for 3 years and been stopped twice (golf) no tickets.
plus its not a full MOT you need, its just a correction at an MOT garage and a stamp on on a bit of paper.