If its taxed, no worries. If its not, get it SORNd
Do NOT forget to do a change of ownership when you sell it.
Yep, you only need to inform them its off road if untaxed. Post the change of ownership docs yourself, or drop them into the office. Not doing so might mean some b*stard drives around in your old car for a time, does loads of minor offences like parking tickets and then sells it on. You then get all the flack because officially youre still the owner.