You would only be able to drive it home legally with a tax disc in the window, so you'd have to put it on a transporter unless:
You know somebody who has legal access to a set of trade plates and would be prepared to use them.
or
You buy it, get the registration document, go to an insurance broker, insure it, take the cover note, MOT and V5 to a Post Office and tax it there
or, if you insure it online on over the phone you must wait until you get documentary proof that it is insured, so you may need to go back a few days later to collect the vehicle
or
Ask the person you're buying it from to tax it himself and pay him the extra (I'm assuming it's on their insurance currently)
Cheers