I would say the minimum MCT for non-connecting flights is 5 hours as that gives enough slack to allow for problems. Any shorter and you run the risk of missing your onward flight and having to buy a new ticket. If your onward flight is long-haul then I'd allow even more time as the costs of getting it wrong are so much higher

For connecting flights I'd allow an hour less as if the onward flight is missed it's the airline's problem

EDIT - 4 hours is the minimum for a T4 transfer as in many cases the traveller would have to travel to the Central Bus Station in order to catch a bus to Gatwick. That could easily add 20-30 minutes to the total transfer time