How to stay a year in Poland when retired.

Ask the uk embassy. For Schengen zone citizens, you can live in Poland without any special permission. You just have to report your presence to the police if you stay for more than 2 or 3  months.

Problem with that is the UK is not in the Schengen zone.  It will be hard, if not impossible for the OP to do what he is hoping to do, one of the penalties of Brexit I'm afraid.

Years ago people, people with Polish ancestors had facilities to settle in the country.

I think I will live for ever in Warsaw now, with a couple of month every year in Paris. It is a very safe town where an average pension in Euros or in Pounds will take you very far indeed. No crowds, almost free transport and nice people, even if they don't always show it. When they smile, it's real! But spending one year here without  knowing the language can be frustrating. Most educated seniors never learned English, just Russian. Rentals are high and property is cheap: For 200.000 £, you will find a large and well equipped flat in the city centre. And a decent restaurant costs 5. We fill our caddy for less than 20.

There are many serviced flats which can be rented for less than 1 year. Just check out on many real estates agencies' websites such Hamilton May(with an English page), although it depends on the city you want to spend time in as short term lets are often located in cities such as Krakow ,Warsaw or Wroclaw.
As for the legal paperwork ,i have no idea what happens for UK citizens now but I would not be too worried about that  or about getting a visa because who in Poland would bother a retired Briton checking his/her visa ? And What for ?
You don't need to work as you are retired, you ll use your money from your own Uk Bank acccount,and if you find a serviced flat you won't have any bills like electricity to pay as water,utilities bills and electricity are included in the rent.
You get a local mobile phone with a pay as you go plan with Tmobile for instance and that's it.

Use friends. I know similar situation - the guy came from Ukraine.

you are  UK  citizen
you live in poland without problem

You can look into getting an Airbnb for housing.