Try to get to the hard shoulder, ideally. I don't think it's a case of 'being allowed to stop'. If your car is going to stop, it's going to stop. It would very much depend on the circumstances. If there's an accident already in lanes 1 & 2 - don't contribute to the problem. If it's clear for several miles and you end up stranded there, probably less of an issue if you've tried to make it to the hard shoulder. But it is illegal to drive in a lane closed by a Red X sign. If you’re caught, you could receive a fixed penalty of up to £100 and three points, and in some cases more severe penalties or a court appearance.

The Gov website has a specific section of advice concerning emergencies and breakdowns on smart motorways: www.gov.uk/guidance/how-to-drive-on-a-smart-motorw...n