The government will be promising to retire the last pacers by 2120!
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Edit, ah beaten to it somewhat with the joke above!
In all seriousness: ticketing is likely to be almost entirely electronic, either through dedicated smartcards or through some sort of all-purpose credit/debit card device which stores purchases on it. This may also be done via what we currently label 'cloud' software/infrastructure.
A larger population will see new lines emerge, most probably outside of the south-east: at some point, the expansion of UK population in south-east in comparison to the rest of the country will become economically untenable. I'd expect to see cities such as Birmingham, Manchester, Glasgow (if still in UK!), Leeds, Nottingham and Bristol likely to see most network expansion.
Some sort of high-speed network, probably consisting of: HS1, the 'Y' HS2, an HS2 Scotland extension, a High-Speed West country route and possibly a High-Speed East Midlands route. Depending on population dynamics, these may be connected up between Birmingham-Bristol and East Midlands-Leeds, but London links will be the priority.