I do think the 30 minute rule is a bit strict. For example, as Marylebone is not the easiest place to get to if say you are on a afternoon shift and have to commute through congested London traffic.
In theory, most people in and around London use public transport but as we've discussed on other threads, you really need to have access to your own vehicle if you work as a driver or conductor because of unsociable start & finishing times.
I used to live & work in Central London in 2006/07 and it is not something I would ever do again. My own theory is you need a comfortable 60 minute commute time just from travelling from another London Borough which might only be ten miles away. That said the Marylebone trainee driver job is a fantastic opportunity for the person who gets it.
CJ