Spanish law does not allow re-registration of rhd commercial vehicles ( or motorcaravans which have windows all around ! ) which is vans & lorries & what are known in the UK as 'car derived vans'. I.E. Vehicles that have windows but are based on a van originally.; Like the Ford tourneo connect, Citroën , Renault Berlingo, Fiat Doblo, etc.
You will see ,occasionally, rhd vans on Spanish plates , mainly around Alicante, both for sale & on the road. These either slipped through due to the tester not putting down that the steering wheel was on the rh side or some other method. ( Like it had come from France where it had been re-registered as they allow it .) What you can say , in all certainty, is that in the event of a serious accident where people are dead/seriously injured & which ends up in court ,is that the vehicle will be declared as 'not able to be re-registered under Spanish law ' & it's Spanish registration will be rescinded , leaving the owner with the possibility of being left with the whole compensation amount , regardless of any blame.
If you are a Spanish resident & are unable to re-register it then you cannot legally drive it even if it is completely legal in the UK. Tax, mot , etc ; as a Spanish citizen & resident foreigner cannot drive a vehicle with foreign plates which they own.