If its a private Sale
and the current owner still has valid tax on it then you can claim you're test driving it (conveniently back to your own house!), assuming you have valid insurance.
But as soon as the change of ownership kicks in, the tax expires and refund goes back to the former keeper. I'm not sure how quickly, technically it would be when you both signed the V5, but as its not in the DVLA system I don't see how it can be checked before the docs reach them.
If its a trade Sale
I don't think there's any reason why the dealer couldn't loan you their trade plate (assuming they have one) to get it home.
I'm in the same boat, looking for a second car that won't be used until the new year, but need it taxed to get it back to the driveway.