Well in case anyone was interested I called Adrian Flux and they have confirmed I am legally allowed to drive other cars that are not insured. The only downsides is that the cover is 3rd party only and it only extends to when I am behind the wheel. It would not be insured at the side of the road for example.

I have requested written confirmation of this by email for proof, but knowing Adrian Flux I’ll have to wait a month if it ever actually arrives at all

All good though and confirms what a lot of people in the insurance industry have been telling me on another forum. The bottom line is it's down to *your* insurance company so ask them!