I think I do mean add, but am sorry if there is any confusion. To take the first example above, CX 251 touchdown 04:40, ETA 05:05. So add about 10 minutes to the touchdown to get a gate arrival time of 04:50, thus this flight was actually at the gate 15 minutes ahead of schedule.

Modern Flight Management systems on the aircraft should be able to deliver it to the entry point to the terminal area (which for flights from Hong Kong will be the Lambourne radio beacon near the M25/M11 junction) with amazing accuracy, like a few seconds, in the most fuel-efficient manner, so there should really be no need to hold for a slot time there. It takes all the routing and weather forecasts and constantly recalculates as you are going along, adjusting speed as you go. But it's still impressive.