I'd check that - clicky

"Many UK policies contain a Driving Other Cars clause which covers motorists to drive someone else's car with third party cover as long as they have the owner's permission.

But many drivers wrongly believe this practice is automatically offered with all fully comprehensive policies. Instead, this can vary between insurers and can take other facts into account including the age of the motorist and their driving history."

I've got my mum on my policy. It cut about £600 off the premium IIRC.

See, told ya.