I have just bought a car that doesn't have any tax on it. It has the V5 document so I will be able to tax it, and an MOT for 6months, but what about getting it home?
What i did when i bought my car was insured myslef in it and drove it home untaxed. You should be ok providing you have the paper work saying you have just bought it. Then stick in on your driveway till you tax it. As this was my only transport i drove the untaxed car to the post office and had all my info on me to prove i was taxing it. I dont know if this is 100% legal but this is what i did.
Go to a Post Office, tax it, then drive it home.
Or hire / find a friend with a trailer to bring it back for you.
The only time you can legally drive an untaxed behicle is on the way to a pre-arranged MOT. I did this once, for just a couple of miles, and had to jump through hoops to prove I had not done anything wrong!
Don't risk it.