Hi,
Just from my understanding, and it of course may be all ok and noted on the policy you future mum in law 'actually' has - but putting her policy to 'any driver' means that any other driver must have their own fully comprehensive insurance policy that's valid in the UK in order to drive the car noted on the policy documents.
Please double check the small print - ignore this of course if indeed it is explicity written on your insurance certificate than you are able to drive or, are indeed eligible to drive and be covered with the details of your personal situation.
Cheers, DtM! West London & Slough UK!