I fly 737. Normal landing flap settings are 30 and 40 deg. Depending on airline policy normally flaps 30 is used due to lower drag (hence fuel consumption and noise). Besides the obvious uses of flaps 40, e.g. short runways etc, Boeing recommends to use flaps 40 for tailwind landings.
It's not uncommon that you plan a flaps 30 landing only to realize there is tailwind during the approach. In such case I normally opt to call for flaps 40. Of course the decision has to be made somewhat before the flare, say above 200 feet.
I am not that familiar with Gatwick, but Wiggy mentioned above that the main runway has a displaced threshold. Hence, when you saw the perimeter fence the height could may well still have been a couple of 100 feet = not too late to make the calling for the next flap setting.
This does not go against the "stabilized approach concept" since it says the aircraft has to be in "landing configuration" - both flaps 30 and 40 are landing configurations, so in my opinion I am allowed to switch to flaps 40 if I desire. At least, I did so more times than I can count and no one ever complained.
I could imagine the same principles would apply for the 777 as well.