Hi Bigman2023
The system automatically generates payments on account when there is a balancing payment of over £1000 and less than 80% of the tax due for year has been collected at source.
Based on your scenario, your UK private pension is not high enough to collect 80% of your total tax due for the year.
The payments on account are correctly charged as you will have a charge each year of roughly the same amount due to the personal allowances remaining at the same level when your income will increase.
Understand your Self Assessment tax bill
Thank you.