As I understand it, driving a vehicle with no MOT and driving a vehicle in a dangerous condition are separate offences. It could pass the MOT today and have a coil spring snap tomorrow. Failing an MOT doesn’t make a vehicle more dangerous; if it has a dangerous fault, it was already illegal to drive it. It just means you drive it knowingly – and IANAL so I don’t know if that makes a difference in this case.
This +1
Driving without and MOT isn’t an offence, you can do it to drive to an IVA or MOT test.
Not sure what would happen if you failed the MOT though, you could fail on emissions, but still be within an older limit, clearly the car is still safe, and arguably roadworthy, it’s just failed it’s MOT.
Rust is usually fine, upto the tester whether they deem it structural or not (within 12″ of a suspension component, sils etc), floorpan I’d have thought would be fine.