I think the situation is a little more complicated than you suggest.
It’s not always the case that there’s an easy choice. A case I read about, an elderly man’s wife died, and his daughter, who was living in Australia with her Australian husband and young children, suggested he went to live there as he no longer had any friends in the UK and of course didn’t see his grandchildren as much as he’d like. I have a similar situation with my granddaughters. That’s not a simple choice to make and not something that can be planned for. I’ve heard of several similar cases as well.
When I first came to Thailand in 2008 to meet the woman who is now my wife there were a lot of things to think about, but my pension wasn’t one of them as I didn’t know how things would work out. Call me an old romantic but I wasn’t going to bring up ridiculous UK pension regulations when deciding whether to live with and marry my then girlfriend. At the time we decided to live in Thailand due to me being 15 years older and my son and daughter being adults whilst her daughter was 13. There wasn’t a firm plan, although I did think I might have to return full time to the UK if my health took a turn for the worse. I’m 70 now but OK so far. I also had in my mind that I’d be spending more time in the UK by now but amongst other things my wife’s mother is getting older so that makes it more difficult. Plans don’t always work out as you intend.
As for knowing the rules, that’s not so simple either. If you pack up and move completely then yes it’s straightforward and also if you move back in the same way. If however you move between the UK and, in my case Thailand then it’s complicated as there aren’t any actual rules as such. I did have a letter from the DWP setting out their method of determining residency but I can’t find it now. They look at whether your flights are return tickets, do you have family, do you have somewhere to live and do you have a car? That’s all I can remember. All departments have their own. HMRC is pretty clear but others use ‘ordinarily resident’ or ‘habitually resident’. I did phone the International Pension Centre early on to ask where I could find the rules. I was told they would be on the .gov.uk website. I said I’d looked and couldn’t find them and was told “They’ll be on there somewhere”. I mentioned this when I wrote to my MP and he couldn’t help either. On various calls when I’ve asked what would constitute becoming resident again, I’ve been told. “Being in the UK for 6 months”. I don’t know if that’s got to be continuous or not. “It’s not really just how long you’re here as it’s based on other things as well that help us decide if you’re resident or not”. On my last call when I pointed out that between early April 2024 and early March 2025 I would have been in the UK for 186 days, so more than 6 months, the woman said “Oh yes that would mean you are resident”. So I’m now resident, at least for now.
It’s not as simple as it sounds is it.