I know every's needs and preferences are different, but I do wonder if the train might not be a better option for you.

As a thought experiment, if you were starting from the city centre and had to check a bag for a 20:10 departure, let's assume you would start your journey at 18:30. Let's also assume that your final destination is somewhere in Central London...

By Plane

18:30 - 19:00 Metro to the airport
19:00 - 19:15 Check in bag
19:15 - 19:30 Clear Security
19:30 - 19:45 Go to gate
19:45 - 20:10 Wait at the gate
20:10 - 21:20 BA 1337
21:20 - 21:30 Walk to baggage reclaim
21:30 - 21:50 Wait for bags
21:50 - 22:00 Walk to Tube
22:00 - 23:00 Tube to final destination

By Train

18:30 - 19:00 Swift one in the pub
19:06 - 22:15 East Coast Mainline service to King's Cross
22:15 - 22:30 Tube to final destination

I've done that route quite a lot over the last few years (albeit to/from Edinburgh) and train wins out for me 90% of the time on comfort and convenience...