I would have focused on electrification from 1955 as quickly as possible, providing 3rd rail for most of the Southern Region and OHLE on main lines on other regions. Where technology wasn't ready/suitable, diesels ordered but the rush wouldn't be to abolish steam prematurely. However, operating costs would have to be reduced where possible while steam was being eradicated.
Additionally, I would abolish the common carrier policy and probably invest in road transport (at least then road traffic profits would go to BR and therefore could support the railway). Close most goods facilities at stations and rely on road-distribution from rail hubs. This would allow rationalisation of trackwork and signalling, although refuge sidings and loops at stations would be priorities for operational flexibility.
I would have started the closure of unprofitable lines, either saving the land or converting to busways. I'd employ some better (read: less biased) metrics for evaluating performance of railway lines. Re-evaluate timetables on basket cases to take advantage of seasonal traffic and commuting/school flows. Where there were lesser-used stations on main lines, I would aim to provide railbus services and provide infrastructure to run these and reduce the number of long(er) distance stopping services. If the station was totally pointless, i.e. miles away from it's town, I would permit closure.
Just spitballing some ideas, go easy on me
