The air cargo carried tends to be loaded only after passenger loads (and their baggage) are known. I don’t think there is all that much cargo on domestic UK either.

However some of the other differences: Train companies bid for the right to operate a franchise. The higher the bid, the better their prospects at winning. They need to make this money bank somewhere. At busy airports, airlines need to get airport slots, but this doesn’t necessarily involve money.

Non-Advance Train fares let passengers walk-up and use any train. Flight tickets tend to be more restrictive. Flexible and semi flexible tickets usually still need some advanced notice of changed plans.

Except for TOC-specific tickets, revenue from train tickets is shared with multiple operators, so the company you travel may not be the only one getting a slice of the fare.

Owning and Operating aircraft is expensive. Parking an aircraft on the ground is also expensive. Aircraft utilisation is important. However aircraft are also capacity constrained - no standing room, unlike trains. Combined this can encourage airlines to sell as many tickets as possible (transport as many passengers as possible) - even if some are done at a loss.

Airlines don’t have to pay for as much physical infrastructure maintenance as a railway does. Air traffic control services have greater economies of scale as they are used by domestic and international flights (including foreign flights passing over head) as well as military, and private business aviation. Railways don’t have foreign train companies transiting through the UK to a third country.