4 hours is a decent guess. When I used to fly into Miami I would generally book a cheap motel by the airport for that night and maybe spend a couple of nights on South BEach on the way back home for a change of pace. reasons being
1) most flights from the UK arrive mid afternoon/early evening US time so by the time you clear customs and immigration it is WAY AFTER MIDNIGHT UK time. If you don't sleep well on planes it's not a great idea to give yourself a 4 hour night drive at the end of a long flight. IMHO grab a bite to eat, get to bed early and leave EARLY before 7AM the next morning to avoid commuter traffic. You're in Orlando before midday the next day which is probably about the time you'd struggle out of you pit had you driven up the night before and not got to bed until after midnight the night before
2) South Beach is a fun area to relax after a busy vacation, it also means that you don't have to worry about getting delayed out of Orlando on your drive to the airport. Even late afternoon flights from Miami mean you have to leave Orlando before 9am in order to build in enough time for any traffic snags as no one likes to risk missing a flight.
It's an easy drive in the daylight when you're well rested, it's not such a nice one when you're tired and it's night time. With the long straight highways in the USA it's REAL EASY to doze off at the wheel if you're tired and that can be a very dangerous thing to have happen at high speed.