Am not saying that the UK rail network is perfect, (and indeed the present strikes make it very imperfect) However, those comparing the costs of rail with air so often do not compare like with like: they take the cheapest advance airfare and compare it with the 'pay at the last minute' train fare.
To make a fair comparison, the comparison should be based on the comparable last minute fare for each method of transport, or , alternatively, at the cheapest available advance fare for each (using an appropriate railcard in the case of trains) . Plus the cost and duration of the transport between the airport and the city centres should also be added to the equation. The two modes of transport come out much closer when you do that .