You can take out "laid up" insurance policies to cover SORNed cars if they're worth it - very cheap, for fire and theft only.
That said, unless you're pretty sure you're likely to use it again, or it's something you love, I'd suggest just getting rid now - the chances are you'll end up doing that eventually anyway without ever getting it back on the road.
I've found with SORNed cars that once they're off the road, sitting there looking an eyesore with everything slowly deteriorating through lack of use, with no tax, test or insurance, they become such a pain to get on the road again, and you adapt to life without it such that you never bother putting it back into action, and end up calling a scrappy to come and crane it away.
If you're not really ready to get rid then it's fine obviously, but you might be surprised how worthless a car quickly looks when it's not in use. A car, after all, is a collection of legal documents and agreements. The metal in front of you, when not accompanied by it's relevant papers, is just a useless tonne and a half of slowly rotting, irrelevant metal filling up your driveway.
I've been there twice, family and friends have done it also. Every time the vehicle in question was in the end either craned away or sold for a few notes on eBay after becoming an annoyance.
