This question already has an answer here:
Two of us holding Indian passports are flying from the Middle East to Toronto with British Airways. There is a layover of about 7 hours at Gatwick airport. The flight lands at the North terminal and the flight to Toronto leaves from the South terminal. We are holding valid Canadian visit visas.
Do we need to obtain a separate transit visa for transferring from the North to the south terminal and boarding a connecting flight in Gatwick?