My take is that best case scenario you’d arrive roughly at the same time you left.

If you have breakfast in London at 8am, then make it to the airport by 8:30, you’re at the gate at 9:30 after one hour of security and controls, and you’ve made it exactly at the time when boarding starts, which usually is 45 minutes before takeoff on most airlines. You take off at 10:15, arrive at 11:45 (which is 6:45 local time), then still have to go through half an hour of border control and getting out of the airport, and then another half an hour to get to the city centre and have a coffee.

You’d still arrive at about 8:30, but I don’t see the whole ordeal taking any less than 5 hours.

I routinely take a 1.5 h flight to visit my family and while I’m a fair bit away from the airport, I don’t think I’ve ever managed to get door-to-door in less than 8 hours. 6 if we are measuring departures lounge to arrivals.