As has been said already - getting a visa for such a move would be a lengthy and expensive proposition. Probably a non-starter. Also, there are a lot of costs associated with living in the USA that we Brits don't necessarily appreciate. Healthcare is very expensive (e.g. I've paid about $15k so far this year), but home insurance in Florida can also be wildly expensive ($10k per year or more in some areas).
My parents enjoyed visiting Florida for <90 days for several winters. They rented houses or mobile homes in vacation communities for each of their trips. They loved it. They bought travel and health insurance in the UK and returned to England for the summer months. You could do two separate 90 day trips a year (as long as they aren't continuous). There are people who stay for 90 days and then visit Canada or Mexico for a weekend to get another 90 days...but they can be susceptible to a zealous US border guard turning them back (it happened to me many years ago and things haven't got any more visitor-friendly). All the best!

Unless my memory fails me, this has never been acceptable. Visits to Canada and Mexico and most of the Caribbean Islands do not reset the 90 day VWP clock.

There is no retirement visa for US.
As said unless you have a couple of million or so for an investment visa or a million for the Gold Visa which Trump is thinking about, you can only visit the US.
If you were lucky enought to meet a nice Floridian man, marry and he then sponsors you, you don't have any other options.

Correct
Government websites eh

You may not apply to extend your stay if you were admitted to the United States in the following categories:

• Visa Waiver Program;

https://www.uscis.gov/visit-the-unit...tend-your-stay

If you are admitted to the United States under the VWP, you may take a short trip to Canada, Mexico, or a nearby island and generally be readmitted to the United States under the VWP for the remainder of the original 90 days granted upon your initial arrival in the United States. Therefore, the length of time of your total stay, including the short trip, must be 90 days or less. See the CBP website. Citizens of VWP countries* who reside in Mexico, Canada, or a nearby island are generally exempted from the requirement to show onward travel to another country* when entering the United States. Learn more on the CBP website.

https://travel.state.gov/content/tra...r-program.html

Any clearer.

My understanding is that after a certain age travel insurance becomes basically impossible as the costs are so high should something go wrong. Also, you have to be careful with the fineprint of how long you have to be at home each year in the UK to qualify, all of which will be checked should there be a substantial claim.

Agreed. It certainly wasn't acceptable as far back as 1995 when a colleague of mine was doing regualar back-to-back 90 VWP trips to DC to work on a project there. He would spend 12 weeks in DC, come to London for a week, then go back to DC for another 12 weeks, a cycle he repeated for at least two years, and I suspect a good bit longer than that! But one time he decided to go and visit the branch office in the Cayman Islands instead of coming to London, and when he returned to DC he was told by CBP that "he hadn't left the US", and was bounced straight onto a flight to London!

Canadian snowbirds usually spend 6 months a year in the US but a Canadian passport allows up to 6 months unlike the ESTA 90 days.

I think practically you might be able to do 6 months provided it isn't subsequent and spend the rest of the time elsewhere. The Caribbean and Mexico are better setup for retirement visas and could be a place you spend the rest of your time.

Health insurance is going to be a major expense. You won't be eligible for any subsidies via the marketplace or Medicare coverage AFAIK.