The DVSA (they in control of the UK MOT system, except Northern Ireland) have just announced that if your cars MOT expires from 30th March 2020 that it will automatically have the recorded expiry date extended by six months and remove the need for owners having to have their cars retested, until hopefully after the pandemic has passed. This will enable owners to re-tax their cars if the renewal date for the tax occurs during this extended period. Owners are still required to ensure their cars remain roadworthy. Click here for the GOV.UK website
Owners of cars whose MOT expire up to and including 29th March do not get the same facility and they will still need to get their cars MOT'd if they are to be able to legally drive, or tax, their cars. Those in this position will have to declare SORN if they can't MOT their car and the tax is due.
Owners of cars that in this period reach 40 years of age and which haven't been significantly changed in the last 30 years can simply do nothing as their cars on reaching that age are automatically seen as MOT exempt.