What will the Government actually do in 2050..?

MAYBE - by 2050:

1. Nation states will only exist to be parts of coalitions on the major global issues: climate change, terrorism, development aid etc.
2. National gov will solely be a mechanism for collecting taxes and passing them to local gov – an administrative task.
3. Local government will be the main interaction point for people (under the huge banner of localism)

This is even before we factor in devolution, the role of the EU and the decline of the western superpowers. 
If I was a politician I'd be looking for another job.