Taken directly from gov.uk website.

If you’ve broken up the vehicle yourself

If you’ve broken up the vehicle yourself you must make a SORN (Statutory Off Road Notification) or continue to tax the vehicle.

You’ll have to make a SORN every year until you’ve taken it to an ATF or told DVLA you no longer have it.

Tell DVLA by sending in your V5C/3 ‘Notification of sale or transfer’ part of your V5C vehicle registration certificate or writing a letter to DVLA explaining you no longer have the vehicle.

In other words, just write to DVLA and tell them you no longer have the vehicle.