I can see this causing a few problems

[rustyrider]

What if the car has no mot. You won't be able to tax it anyway, so the rule of being allowed to go only to the test station is fucked as there will be no tax either, its all a con

It won't make any difference.  At the moment you don't need a car to be taxed to take it for a pre-booked MoT, only insured.  Think about it, if it isn't MoT'd you can't tax it anyway.  I'm involved in importing cars and take them for MoT with no tax or number plates which is perfectly legal as long as they are insured.  Once they have an MoT I can then apply to register and tax them.

The other thing that they are doing is allowing you to tax a vehicle monthly by direct debit.  Now in that case I suspect there would be a grace period so you can buy a car (or bike) drive it home, covered by the tax paid by the seller which I would assume he will still be liable for until he sends the V5 in to notify a change of keeper, and as long as the new owner sets up a direct debit in the same month I can't see it being a problem.

What I can see being a problem is with pre 1973 historic vehicles which currently have to have a tax disc but it is free.  You can't drive a vehicle without tax and you can't have a direct debit for £0.00 paid every month so what is to say that they are taxed or not?