Payments on account are two advance instalments towards next year's tax bill, each 50% of last year's tax, due on 31 January and 31 July. You make them if your last bill was over GBP 1,000 and less than 80% of your tax was collected at source. In your first profitable year this can mean up to 150% of one year's tax due at once.
What they are - Two advance instalments towards next year's tax bill, paid in addition to settling...
How much each is - Half (50%) of the total tax you owed last year
First instalment due - 31 January, alongside the balancing payment for the year just ended
Second instalment due - 31 July
Plus 6 more in the full guide

freedomisntfree.co.uk/guides/payments-on-account-uk