im not sure if the smaller post offices are able to do it due to not having the live system, it is also possible to do it on line via a V70 form (which states you can’t do it at post office) but since all the dvla local offices shut 5/6 years ago I’m sure the main post offices are able to do it, if you don’t have the log book you may also need the V62 form (£25 fee).
https://www.gov.uk/government/publications/application-to-change-a-vehicle-tax-v70