Driving without tax may invalidate your insurance.
Never heard of this being the case, and I strongly doubt it. Unless your insurance company has a clause stating this, I think they'd find it very hard to uphold this as a reason for refusing a claim if the car has a valid MOT and is otherwise road legal.
It's not the insurance companies' job to police the laws of taxation.
However, if you are prosecuted for driving without a Road Fund License you will normally be required to pay the "back tax" plus a fine which, the last time I was convicted, was 1.5 times the "back tax". Plus costs. No points on your license.
This is true, it's not an endorsable offence so you'll never get points, just a potentially nasty fine.
However, if you purchase tax for January this is enough for DVLA and you should be OK. In my experience. I am a good lad now but "in my experience" is more times than I have fingers to count with.
I would drive it (did the same with my MV6), but don't have an accident!
Jim
I have done it a few times too, purely because it's a complete and utter pain in the arse to get tax for a car you've just bought. Insurance company often won't cover it until you've actually taken ownership of the car so you can't get the certificate in advance, meaning you simply can't get the tax for a few days after purchase.
Not sure I'd do it now though, too many ANPR systems around. A few days after I bought the Omega I put tax on it and was driving in Glasgow. Out of the blue a police car appeared and pulled me over, before even speaking to me the first thing one of the guys done was look at the tax disc, before saying to his colleague "oh the tax on it is valid" in a surprised tone. I expect it showed up on the PNC as having no tax, as the previous month it was untaxed and the records probably hadn't been updated since I bought the tax.
I just wish there was a way to buy temporary tax, allowing you to get a car home legally, without needing all the documents that are impossible to get at short notice. For example they could charge £10 or even £20 for a one day tax disc that can be purchased without showing insurance cert -- of course the car still has to be insured to be legal, but it facilitates those cases where it's not possible to get a certificate in time. At an equivalent of £3650+ per year somehow I doubt uninsured drivers will use it as a loophole to avoid showing a certificate!