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Of course it's a payment in advance, you are making it in advance of your tax calculation being made. The January payment is made on the assumption that you will earn the same amount as you did the previous year not actual earnings since April. You can also file and pay your tax on the 6th of April if you wanted to.
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The January payment is made on the assumption that you will earn the same amount as you did the previous year
And you can tell them you're not going to make as much and they will reduce your payments on account.
It's only a payment in advance if your work is highly seasonal and you do less than half your annual profits between April and January.
Of course, all bets are off if your basis period isn't April to April 🙂
It's not technically in advance, since your January 2019 payment on account is tax on work done since April 2018.
Also if your outstanding tax for self employment is as little as that and you have an employment too, you can choose to have it taken over the year with your PAYE tax by having your code adjusted.