UK would benefit from pensioner visas like many Latin American countries have. Pensioner's do not take jobs, spend, and are generally less troublesome than younger visitors. And they're generally done having children. The biggest drawback might be use of your NHS, which they could pay into as citizens do. The LATAM countries generally have minimum income requirements for pensioners.
Even within the USA, pensioners tend to move around to places with lower cost of living. Where I live, we have mild winters, if that. We also have a low cost of living given our proximity to Mexico. They'll drive the 1000 miles or more to get here every autumn to winter in a second home. They save on energy from not having to heat their homes. They don't bother with snow. And they are less likely to get winter illnesses. It's worth the journey.
Likewise, arbitrage could make the UK a popular destination for pensioners from the US who are looking for less costly places to visit. I doubt they would winter there. They would more likely stay in villages than large cities. They could largely do this with a full 6 month tourist visa. But these tend to be too restrictive for people who largely don't have an itinerary for their stays.
Interesting point, although the last thing we need is more pressure from the global rich on house prices!
Possibly. Our Winter Texans (that's what we call them), mostly live in park homes or age 55+ communities. Sometimes the park homes are in 55+ communities. This housing is generally unavailable to most locals. As Winter Texans inevitably die, their children sell off the property to another Winter Texan, or the children age into becoming Winter Texans themselves. These aren't wealthy people. Our municipalities generally welcome them and have events for them as they spend millions, in aggregate, during their visiting season. Our local university even conducts studies on the economic impact. It's more like an industry than an invasion.