What makes you think that it was ‘mileage’ rather than ‘losses’?
A lot of closures were of individual stations on lines that retained passenger services. Many other cases were withdrawal of loss-making passenger services on lines that remained open for freight, especially in industrial areas.
I am genuinely interested in where your ‘fundamentalism’ comes from.
As an example, the Swanage line, where the manager at the time reportedly said to Barry Doe that he'd wished he could have closed the Lymington branch instead as the Swanage line generated more revenue. This clearly suggests that there was pressure to get rid of route milage.