When is residency, residency?

[NomoneyNohoney]

Coming to this a bit late, but want to add my own situation.

I am dual-nationality, UK and one non-EU nationality.
When I enter the UK, I use my British passport, same when I leave.

When I enter my other country, I show the passport of that country.
Usually, the foreign immigration officials potter back and forth, and then ask to see my British passport (which I'd not mentioned to them.)
They invariably put a foreign entry stamp into both passports, and that's that.

My guess i that simply, they get a list sent to them by fax or email, of names, passport numbers and nationalities on incoming flights, and then realise that I'm showing a different one to that which I used to exit UK. They stamp both passports, and everyone is happy.

That passenger list they email, could well be a long-term record of arrivals and departures. If I recall the early days, to be non-resident in the UK, you had to be outside the UK for minimum 270 days per tax year, and in the UK for maximum 90 days per tax years.