Was due to fly LGW to CUN in May in Business, but received an email this morning to say that our outbound flight had been cancelled and they had put us on the flight going out the next day. As this is a surprise birthday present for OH, I had already booked the accommodation in CUN + Sofitel Gatwick to stay the night before flight + car parking in the short stay (easy access to Sofitel and terminal). Just rang BA and they have now got us on a flight leaving the day we were supposed to fly, but its leaving from LHR Terminal 5 and it's LHR-MIA-CUN. I'm not changing the Sofitel Gatwick booking and will also still be leaving the car at Gatwick.

My questions are:

a) which is the best way to transfer from Gatwick North to Heathrow Terminal 5 + how long does it roughly take (our flight is in the afternoon).

b) which is the best lounge to use at Heathrow Terminal 5

c) as the flight to MIA is a 4-class plane, with a Business Upper Deck - which are the best seats to try and get.

d) we have a 2 hour wait at MIA so will we be able to use the lounge, and if there is more than one, which is the best to use.

Thanks