Of course SORN is automatically cancelled when you tax the car.

When you tax it on line you will get an e-mail (to your smart phone if its set up to access e-mails) which will show its been taxed and this should be proof enough for the plod if thier computer has not updated.

But be aware that to tax a car the MOT status is not checked but if you were pulled by plod and had not MOT you would get a suitable fine (up to £1000). Normally there are no points but if the car is classified as "Dangerous" its up to £2500 and 3 points.

With the current 6 month extension the MOT status may be irrelevant anyway.

As long as you have the registration document with the documents number you can tax it. It will automatically come off sorn. If driving home. Have a copy of the email confirmation and then you should be OK.