I'm not so sure about this - many HGV trailers are left at the side of the road but they themselves are not usually covered by an insurance policy as such
The HGV's insurance covers the trailers when in use, and when decoupled they aren't a motor vehicle so I guess wouldn't be covered under the RTA.
http://www.nopenaltypoints.co.uk/unused-car-parked-street-does-need-insurance.html