Flybe has been running radio ads in Ireland in recent days promoting the DUB-LCY service. The key USPs they are touting are "the only London airport that's actually in London" and "you can go from plane to taxi within 20 minutes, and be in Liverpool St 20 minutes after that." The "plane to taxi within 20 minutes" seems to me to be underselling the point of LCY pretty comprehensively. If you have no checked luggage, you can go from plane to taxi within 20 minutes at Gatwick or Heathrow (or 2 minutes from a domestic UK/CTA flight at LCY), so you are unlikely to be particularly impressed by this claim for LCY. If we are to understand that the 20 minutes relates to retrieving a checked bag, it might help to specify that (unless that also requires flybe to add T&Cs to the effect that checked baggage costs extra?).
The difference at Gatwick and Heathrow is (1) the time taken to go through depends on the gate and the queues (if not domestic), and (2) the length of time spent in the taxi on way to Liverpool Street.
Incidentally, Heathrow is in London as well. London Borough of Hillingdon.
Pedants corner: think BE is referring to
geographical London in its ads, and not referring to any administrative arrangements.
Doubt if BE would use the same comments if it had any Heathrow routes and didn't do so for its former Gatwick routes.
It's doubtful that anyone who lands at Heathrow (or Gatwick, Luton, Southend, Stansted) believes that they are "
actually in London", to quote the BE ad, especially those who have actually done the journey up to London!