@FOJN
we simply don't know how many people come to work and then leave.
Yes, it's true that the data's bad. A crude reading of the attached though suggets about 40% return home (200k leave /420k arrive, annual average. The true % should be higher as immigration was rising during the period in question, once we consider the time lag).
It also varies a lot by country of origin, so it's within our policy control to a large extent (I wouldn't be Draconian about people coming here to retire, but I don't think it's a moral imperative to treat all immigration the same either). What I'd actually do is have more types of visa, some with very restrictive rights (e.g. no settlement) that are easier to get than the fuller versions, then let people freely choose what deal they want to apply for.
https://migrationobservatory.ox.ac.uk/resources/briefings/permanent-or-temporary-how-long-do-migrants-stay-in-the-uk/
We do know that in the last 20 years net immigration has increased and our population size has steadily increased in a way that cannot be accounted for by an increased birth rate or dramatically increased life expectancy.
Yes, so in crude terms I think that's a good thing (shrinking or aging countries are not powerful ones, quite apart from the economics/freedom/humanitarian stuff that may or may not persuade you). I think housing shortage is a big problem, for which I mainly blame planning law and our voting system (property owners vote) not immigration. In other words, even absent any immigration, I'd favour much more housing, so to me it's the housing supply that's the problem not the immigration demand. For other public services: goes back to the public finances argument. We are better able to afford good public services with more immigration. We just haven't chosen to do so.
People who meet qualifying criteria can also claim their UK state pension when they return to their home country, others will stay and make the UK their home.
Yes, true. I'm happy so long as the numbers still work out favourably, which from all the data I've seen they do. Also, future retirees won't get anywhere near as good a deal as present ones, which is another reason it's not a Ponzi scheme even aside from headcount. Basically we have an state retirement offering at the moment that isn't fully covered by current retirees' past contributions, so we need to plug a gap.
I think it's also worth considering that many people who come here to work send money home.
With my humanitarian hat, this is a good thing. And with my UK economics hat, it's still a good thing even if this amount were 100% (because freely agreed trade is good for both sides, so the benefit has already happened. Yes, more would happen in the UK if that money were to stay here, but even if 100% of the immigrant's side of the deal doesn't stay (which is an unrealistic extreme) it would not negate the economic benefit.
The effect on labour competition is a harder one. Immigration is beneficial overall, but there are certainly localised harmful effects for some workers already here who get competed against, This is why I think immigration proponents need some humility --- not everyone will benefit, even though most will, and the average will. And it's rarely the people like me who are going to be the most dislocated. But, as before, bigger pie much better than smaller pie, when trying to come up with distributions under which everyone benefits.
Similar for culture: while I think all the arguments I've deployed until now have moral force, there's another one which is purely aesthetic, which is about preference for mono-culture or multi-culture. I can't see how either preference is morally better than the other, but way too often one is seen as good and the other bad. I like hearing lots of languages on the bus. That doesn't make me better or worse morally than someone who dislikes it.
(There are other cultural issues, but I'm pretty certain that you and I agree there are harmful and beneficial bits of culture, and we should try to limit the harmful imports. I do worry about this, but less than might seem intuitive as data, from the US, seems to show that the host culture changes the immigrant culture much more than vice versa. Sadly that means 'good' cultural imports get diluted, but so do bad ones. So, except for extreme examples, this isn't a huge worry for me).