If you have recently started trading as a self-employed person, your first income tax bill will be due in the January following your first trading year, and for many it comes as a bit of a shock.  

This is because your first tax bill, due in the January, will commonly include an extra 50% as a Payment On Account (‘POA’) towards the tax bill for the following year.  In addition to this, a further POA of 50% will need to be made the following July.  So, within a six month period, you may well have to pay two years’ worth of income tax! 

For example: If, in your first year of self-employment, your tax bill, based on your 2017/18 profits is £2,000, then you will have to pay tax of £3,000 (£2,000 plus £1,000 POA towards 2018/19) in January 2019. Then in July 2019, you will have to pay tax of £1,000 (the second POA towards 2018/19). 

Thereafter, there will effectively be six months’ worth of tax payable every six months – and a further balancing payment (or possibly a refund) every January.   

Also, because of the way POA’s are calculated and paid, self-employed people can often find their tax bills don’t seem to be in sync with their income.  In contrast, with Limited Companies, Corporation Tax is always payable 9 months after an accounting year end.  

http://www.ams-accountancy.co.uk/blog-post/explaining-hmrc-payments-account-january-and-jul