Heathrow is the standard alternate for Gatwick on all fleets at Thomson.
There are fuel alternates for legal planning purposes and alternates for company diversions. I suspect LHR is the former; but I would expect the captain to be in contact with the handling agent at LGW and execute company instructions. The responsibility of gaining permission from LHR would be on mission control not the crew. "I was only obeying orders, guv," comes to mind.