I looked at one way morning fares for Leeds to London by train, it’s £143 tomorrow, £125 on Monday with a few £88s around in three weeks time. The last minute ones come with warnings of limited seats being free. Evening journeys north start from £104 tomorrow and around £59 in a months time.
There probably is a decent market for those wanting to go south in relative comfort and those wanting to save a bit of money. It’s worth noting anything less than £143 single on the train is no more flexible than the flight as it’s set train only. Although the airport comes with costs like parking, city centre car parks near stations aren’t cheap either.
The problem will be filling the opposite direction where the train at equivalent times can be as little as £38 return. And of course whether undercutting the train even at something like £200 return makes money with LHR’s charges.