owning a car without a current MOT is not a crime, driving on a public road in a car without a current MOT is, unless you are taking the car to a pre booked MOT. if you have been caught by an ANPR camera you may be prosecuted, get the car booked for an MOT as soon as possible
A few years ago after a friend of mine was stopped and given a form to produce documents he discovered his MOT was about a week overdue. He immediately got a new MOT and took both new and old to the police station and he was let off with a warning.
MOT expired but still have tax -
It might be the maximum, but you'll get a small fine (£60, no "points"), unless your vehicle is unroadworthy (not just an MOT failure, but *unroadworthy*, which is different), in which case probably more.