Tedious travel query please

How long would it actually take to get from the dead centre of Edinburgh to Central London (let's say Trafalgar Square) by plane? Hand luggage only.

I know it's about 5 hours by train, (15 minutes to station, 4:20 on train, 20 mins from Kings X to wherever) so I'm trying to work out how much time I'd actually save by flying and whether it's worthwhile.