Hi,
Payments on account occur, when two criteria are met.
There is an self assessment underpayment of tax of more than £1000.00 and less than 80% of the unpaid tax can be collected automatically (such as via a tax code operated by an employer).
If both criteria are met, then the unpaid tax for the year, is used as a starting point for the next tax year, to create payments on account.
Payments on account allow you to pay the tax due for the next tax year in January and July, with any shortfall payable as a balancing payment.
More information can be found at:
Payment of tax: payments on account
Thank you.