How do I tax a disabled car when purchasing as this must be done by post?

My son has disabled tax status on his car. Soon we will be part exchanging for another car. How do we arrange the new tax to drive home as disabled status can only be done at a post office? Do we just...

Can I get a discount on car tax if I receive PIP?

Can car owner who receive PIP benefit claim a reduced car tax?

If I am disabled do I get a 50% discount on new car registration tax?

I'll be getting delivery of a new car in January and was wondering if you could tell me whether I can get a reduction on the first registration tax through being disabled. I normally get a get a 50% reduction...