There is no clear data on this. It's not even certain how many British
nationals are living in other countries:
The data on the total number of expatriate Britons is also uncertain: the IPPR
found 5.5 million Britons live abroad, while the World Bank puts the total at
Most sources report the 5.5 million number though (which, of course, doesn't
mean it's true).
Sources differ even more on the the number of people who haven't lived in the
U.K. for more than 15 years, and most of them acknowledge that there's no easy
way to get this figure.
The 2 million figure is by far the most common, but I don't know what the source
of that number is (and "up to 2M", "roughly 2M", and "more than 2M" are all quite different). It could just be that this number is the result of news
outlets parroting each other, which is not uncommon with these sort of "soft