Plane definitely does not win hands down versus train.
In recent years, domestic air travel has fallen significantly. The proposed HS2 line would give additional capacity to the MAN route (when it eventually gets extended), as well as providing passengers from BHX - the bread and butter of the route.
The majority of people will check in 2 hours beforehand; it takes 1.5hrs to get to LHR from EDI/GLA, you have to wait for your bags at the other end and then travel onwards to your final destination. LHR takes about 30-60 minutes to get into Central London by tube/LHR express.
There needs to be the choice and competition between the two modes however.