The answer depends on your circumstances and your vehicle. Your vehicle must have a valid MOT certificate, or your insurance could be invalidated.
If your MOT was due on or before March 29th 2020, you must ensure this is still carried out. Garages undertaking MOT tests have been deemed essential, so can stay open during the lockdown.
If your MOT is due on or after March 30th 2020, the DVSA has announced that all cars, van and motorcycles will have their MOT extended by 6 months. This means your certificate will remain valid for this extra amount of time and won't invalidate your insurance policy. However, your vehicle must be kept in a roadworthy condition and drivers can still be prosecuted if they drive an unsafe vehicle.
You can read here regarding MOTs due before March 30th 2020, and here regarding MOTs due after 30th March 2020.
If you live in Northern Ireland, you can instead apply for the Temporary Exemption Certificate (TEC). Please see here for details.