It may do. From a time perspective, you could argue the time taken to fly LBA-LHR (1hr?) but then all the waiting in queue, then transferring into London makes it look pointless.
Not everyone taking the flight is heading up to London, those that are may prefer the train, but for those heading for the Thames valley and other points south or west of London, flying may well be much more convenient.
As for those with onward connections, it's a no-brainer!