It's a SH*T idea. I have a couple of classics, I don't run the insurance and tax at the same time, that way if I want to run it for the MOT test I can do that before the tax becomes due, if it needs work I can take my time and have it SORN, but this now means i's have to remember when the insurance of a car runs out, which would be different to the tax, or else I'd get a fine.