i always thought the car had to be taxed.
but when my old esccos got written off it took months for it all to go thru on the insurance side - they wouldnt allow me to transfer the plate, but when they eventually gave me the right to take it off my tax and also MOT had expired, i wrote to the DVLA, they just sent smeone round to chat to me and then allowed me to put the plate onto retention.
sorry, long winded there