It could be where they're living. In Italy, you have to have a proper rental contract or own a house to apply for residency. I suspect that these people might not have had either.Me too, apparently they were laughable. 5,000 euros a year income and health coverage and minor paperwork.
Could be possible that if you are resident in Spain you need to file taxes over your retiree funds in UK, as they would not be retiree funds in Spain but income as they don't care about the source?
But I don't think many people will be legally kicked out from Spain, the agreement reached between countries covered all the current residence in both countries (and the other countries) to not disrupt their lives
My occupational pension from the NHS isn't counted here as income for our Italian tax returns, as it's a government-paid pension (I'm not old enough for a state pension in the UK). State pensions from the UK wouldn't be double-taxed, either.