they renew it. We don't require you to renounce your other citizenship.
And you have to provide the certificates for your parents births, deaths and marriage.
They might even ask for your grandparents details as well.
Clearly, if your parents or grandparents escaped a country during difficult times, there will be difficulties in acquiring the certificates, especially if your parents are now deceased.
If they didn't escape with you, or in cases of obtaining your grandparents details, it will be almost impossible.
The daft thing is, if you're not able to provide the information required, and you had only the one passport (UK), and they refuse to renew it, you're stuck in the UK and can't travel anywhere.
In the case of people having had just a UK passport, what's the point?
In the case of dual passport holders, it's obvious why it is happening, as MBK said, it's an easy route to remove your UK passport, just for not being able to provide the details requested, no 'naughtiness' required.
Take for example, British Army veterans from places like Kenya (escaped in 1963), Uganda (escaped in 1972), refugees from India, Pakistan, and Bangladesh (escaped in various waves between 1947 and 1970), Cypriots (escaped Cyprus 1950s and 1960s), and many of the Windrush generation from the Caribbean Commonwealth countries, (1948-1971)
All of these (about 200,000 not including the Windrush generation) have lived, worked, married, had children, probably now drawing their state pension, are having to go back decades to find details about their parents, and grandparents, who may or may not have escaped with them.
Refugees from those previously UK colonies, now have to apply to various administrative centres in these other countries, from UK, for the certificates of their parents and grandparents, if they exist.
And it will impact their children also, if/when they apply for their passports.
The alternative is acquiring citizenship. Currently it costs over £2,000 for a citizenship, if they can't find the details for their parents or grandparents.
When the veterans were demobbed, they could have it for free in the first two years of their de-mob. Now they have to pay, and it will take months, and much correspondence between the UK, and various other ex-UK colonies, at additional costs, to complete the process, not to mention the subsequent verifying of the details by UK civil servants.
Nepalese veterans were excused such processes, eventually, after a much publicised campaign. It should apply to all British Army veterans born abroad.
I'm seriously considering emigrating. I don't like the direction UK is going.
Of the obvious English speaking countries some offer easy retirement visas.
South Africa requires an annual income of £12,000, Mauritius require a $18,000 annual income.
Uganda offers retiree visas for those with $26,000 dollars annual income. Kenya offers a similar visa for those with $36,000 annual income.
Thailand requires £1,500 monthly income, Philipines with $10,000 deposit and $850 per month income.
European countries (especially the EU ones) are (or have already) in the process of making it difficult for GB retirees, usually requiring obtaining citizenship, which invariably requires a language test.