There are probably a million overseas British nationals, plus British immigrants, and their children, and grandchildren, and wannabe Brits in India, Hong Kong, the Caribbean, Indian Ocean, and Pacific.
There are going to be a lot of empty houses, factories, schools, etc.
The infected will be dead in a month or so.
Figure on Britain being pretty much functioning in a year, but mostly village based because the cities will be gone to fires. Population about ten million and an economy based on tourism and agricultural exports.