I was always under the impression that you cannot whatsoever drive a vehicle with no tax on a public road, and that it would have to be put on a trailer, even to and from an m.o.t. And driving to and from an m.o.t with no m.o.t you can drive back home or to a garage for repair. Only reason i say this is because last august i bought an old corsa to nip about in (so i didn't have to cancel my insurance and lose my no claims) and i drove it about 10 miles home with no tax, but i did insure it there and then. I passed traffic police too! When i got the car home i checked on the dvla site, and it said driving without tax is illegal no matter what, even driving to and from booked m.o.t's!
Not saying this is true but after panicking about how i could have been pulled and fined, i was told that in most cases the police aren't too arsed about tax as it's not really a police matter, it's more in the way of the dvla to sort out. Though the police can do you for failing to display!
Also to clear up another post on here, if you tax your car up to a day before it expires, you can legally use your car without displaying a tax disc for 5 days after expiry :)