Although the deadline for tax returns is not until 31st January, if you file it online by 30th December and the amount of tax you owe is less than £3000, you can have the tax that you owe collected through your PAYE code, instead of having to pay it in one lump sum by 31st January.
There are many considerable cash-flow advantages to doing this.
Henry files his tax returns online on the 15th December. He pays £0% tax and owes £1800. He chooses to have the tax collected through his PAYE code. This delays payments, spreading it throughout the following tax year and saves him from having to make a payments of £1800 by 31st January.