I don't think the difference is as big as you think it is e.g Central London to Central Glasgow.

Tube 30mins -> Euston 4h15m -> Glasgow Central -> 10m cab ride = 4h55m

Tube 30mins -> Liverpool St 45m -> Stanstead 1h -> Fly 1hr -> security/reclaim 30mins -> 20min cab ride =4h05m

But on the train you get 4 hrs uninterrupted work time and first class is often only £10-20 extra and gets you unlimited free food, booze and wifi. Way better value than a BA flight