retiring to nairobi 71 years old

You could apply for one of the various permits...

Such as:-

Application for Permit Class K: (Ordinary residents(KEP/K).)
This permit is issued to a person who;

    Is not less that thirty-five years of age
    Has in his own right and at his full and free disposition an assured annual income of not less than USD 24,000 amount that is derived from sources other than any such employment, occupation, trade, business or profession as is referred to in the description of any of the classes specified in this Schedule, and being an income that either; Is derived from sources outside, and will be remitted to Kenya; or Is derived from pension or annuity payable from, sources in Kenya/li>
    Undertakes not to accept employment, paid or unpaid, or engage in any income generating activity of any kind without a permit of the relevant class;
    Whose presence in Kenya will be of benefit to Kenya


Read through the different posts on visas and permits to get to know what would suit you.

If you can invest the money from the sale of your home, to top up your state pension, so that you have an income of at least $24,000, then you can apply for a Class K Permit.  Of course Brexit and the associated devaluing of the pound is making this a more challenging goal.  Unless you are married to a Kenyan, this is probably the only route open to you, barring dividing time between Kenya and the UK, on serial Single Entry Visas.

It should be noted that your income can be derived from investment income from 'home', or Kenya.  It's worth bearing in mind that the interest rate for investment accounts, in Kenya is around 6 - 8% (roughly).  However, opening an account is trickier than it used to be, due to international money laundering laws.  Also, the Kenyan government do like foreign sources of income to boost the GDP, which in turn makes the "Whose presence in Kenya will be of benefit to Kenya" requirement a bit of a 'no brainer'.

Bear in mind that the Class K Permit costs around £1600 and is valid for 2 years, so your source of income in Kenya must be long term and secure.  A lump sum is not acceptable as proof of income, neither would it be wise to obtain your permit, then spend your investment on a house, car and so on.

General advice; if you have any long term health problems, consider your options very carefully indeed.  Medical care tends to be money and greed driven.  Doctors are frequently arrogant and the concept that the patient is, to an extent an expert in their own health situation is totally lost on most of them.