At 40 years old a car becomes MOT exempt (1980) so officially you need not have to have the annual test or if you do get one as a safety check the vin number does not require checking.
The 40 year rule is a rolling law so next year it will be 1981.
To get exemption from the MoT you'll need to register as a Vehicle of Historic Interest (VHI) When taxing your car, you'll need to declare that it is exempt from the MoT process, and confirm that it has not been substantially changed in the past 30 years. A chassis change will or may disqualify a vehicle depending on when it was carried out.
https://www.adrianflux.co.uk/blog/2019/07/cars-become-mot-exempt-hit-40-years-age.html

I may well be wrong here but I'm sure I remember that some left the assembly line without the vin stamped on the chassis.