Applying for UK Vehicle Tax in advance
I recently attempted to retax our camper van, this was on the 27th April, I wanted to start the new tax period from the 1st of May but the online system forced me to backdate the tax period from the 1st of April, making me pay for almost a full month for which I would not have used the vehicle. Nowhere on the UK vehicle licensing site was there any clues as to how far in advance you could tax a vehicle.
Eventually I found this information, so if it is any help to others, this is the answer:
If your vehicle is already taxed
You can apply from the 5th day of the month in which your current tax disc runs out. Your new tax disc will come into force on the first day of the next month and you should not display it on your vehicle until then.
If you are going to be abroad when your current tax disc runs out, you can take or post your application up to two calendar months before your current tax disc runs out to a Post Office branch that deal with advance applications. If you apply by post, you must send a letter explaining why you are applying early.
If your vehicle is not taxed or has a SORN
If you apply in the last two working days of a month, the tax disc can start on the first day of the next month. If you apply before then, your tax disc will be backdated to the start of the current month. (Saturday is classed as a working day at Post Office branches.)
Tax discs are no longer issued in the United Kingdom (UK)
The DVLA no longer issues tax discs. The information is held electronically and can be checked by relevant suthorites as required.