I've read somewhere both M0 east and M31 have space for six lanes, but that doesn't seem to be the case.
Usually, additional space for a future widening is reserved in the median, so that if they want to widen it, they don't have to adjust all bridges and overpasses. They did this a lot in the Netherlands. The Dutch road authority is smart, if major maintenance is needed, they sometimes prepare a 2x2 motorway for 2x3 lanes. For example, the A28 near Zwolle is currently being widened, they widened 7 km of motorway to 2x3 lanes in only 10 weeks.