The median should probably be used - the trains are much more likely to be used by people at the lower end of the wage scale. Charles may live in London and make ungodly income, but he’s unlikely to be found on the train.
I don't think anyone commutes from the Home Counties to a job at Pret. People on lower income still live in London, just in not as great housing. People on the train are people who can afford the choice of a luxury of a home in a village or the countryside instead of London.
The "Stockbroker Belt" is a thing, and Sir Humphrey used to commute in from Haslemere (if only fictionally, but I'm sure he was based on real examples).
