Yes SP may be below the basic personal allowance
but those pensioners who have a small occupational pension will be dragged into paying tax. However, their occupational pensions are so small that the tax due cannot be collected via PAYE also they will receive assessments after the end of the tax year.
Between 2022-23 and 2023-24, HMRC figures suggest that the number of those aged 65 and over who pay income tax rose from 7.73m to 8.5m, due to the 10.1% increase in the state pension in April 2023.
There are articles in this, one here
https://www.theguardian.com/money/20...-blue-tax-bill