Originally Posted by
OxfordBlue
But... the OP has a visa for her final destination, and can prove she has a booking to there by showing a printed itinerary, and can transit airside at LGW according to the airport's own rules and doesn't require a DATV.. so if there's any nonsense at the airport, printed copies of the relevant regulations should suffice.
Well, that would be up to the check in agent to determine. As far as they are concerned, they are transporting the passenger to a destination where she does not have a proper entry papers. If they chose to be pragmatic about it, on the grounds you mention, it could work. But I would say there's a chance she will be denied boarding.
My advise then is to get the proper visa for the UK, just in case. With a Spanish residence card, it's usually a matter of formality to get a visitors visa to the UK