I don't think there is any official definition, but "expat" to me implies someone who either plans to return or lives in an ethnic/linguistic enclave and doesn't integrate to any extent. I consider myself an immigrant.
https://www.economist.com/britain/2014/08/07/and-dont-come-back
Some of the more aggressive responses on this thread are reminding me of this article by the Economist. Sorry, paywalled, but essentially it makes the point that
a) numbers of elderly Brits retiring to places like Spain have fallen a lot since 2006 and these people are no longer very representative of British people who live overseas
b) Britainthe authorities, that is is notable for how little it engages with its emigrants. Some countries grasp that emigrants are a valuable tool for building up connections of various kinds that can enrich the home country (and can bring back new insights and new skills if they decide to return), and go out of their way to engage with their emigrant population. The British authorities tend to view a decision to leave as a sort of zero-sum decision/insult to the country, and adopt a generally sour attitude. Judging by some of the nastier posts on here, it looks like at least some of the population shares this view.
Voting is more likely to shift a country in a positive direction when more voters have wide and varied life experiences (which could be all kinds of things including various kinds of jobs, travel, education and so on). Overseas voters may have particular insights which come from the experience of living abroad and it can be helpful to be in a position to stand back and view one's country in this way. You're more likely to value things like improving cities' public transport, for example, if you have experience of living in cities where public transport is good and have been able to see how much it improves productivity. You're also more likely to appreciate good things about the UK which can be hard to notice from close up - the UK is unusually good at integrating people of different races, for example, from my perspective.