i would get him to take it to the local DVLA office and have the plate taken off BEFORE you buy the car. i once bought one where the previous owner had to take his plate off and he messed around, and i waited months for the docs.

although i have sold a car myself which still had my plate on it, and there wasnt time for it to be took off so he took the car and agreed to wait for the docs (he knew me so knew i wasnt pulling a flanker, i gave him photocopies of the docs). only took about 2 weeks then i got the proper docs and posted them to him (recorded delivery).

i had already given him the original number plates of the car ready to swap over when it was all ready (so he didnt have to go and spend £20 on new plates).

if you both know/trust each other its fine but beware some people will fook you about and not send the forms off for ages.

if the car doesnt have much tax left on it, tell him to fook off and sort the plate out now or you will end up with a car with no tax and no way of taxing it for months (thats what happened to me, grrr!)