I've asked this question before ........ and not sure you're correct here
the fact that its off a public highway or not as far as road tax is concerned is irrelevant other than you then can't take it on a public road ...because you have no insurance - so because you would have no insurance, there is no need to not have it road taxed. This as far as I can see works both ways round
unless you've found something else to confirm otherwise ?
I await to be knocked firmly back down and replaced back in my box
re : theft and damage etc - no question - if you want to minimise your risk then, yes you have to insure it