it will say on the inside cover classification that it's disabled which is what you need to get changed, hence you may not be able to do this at the post office

after that you will then need to actually tax it so you have 2 separate issues to contend with, but you've got to get it unregistered first before you can tax it

it took me 2 weeks to sort mine out and, if i'm honest, had i known it was going to be such a pain in the arse i would have paid extra to buy the tax and then driven around for those 2 weeks until it got sorted