There's is an earlier deadline for Self Assessment of 30 December if you want HMRC to collect any tax you owe through your tax code. You can ask for this if you owe less than £3,000. Please show this clearly on your tax return. HMRC will try to collect the tax due through your code, but they can't always do so.
Remember now is the time to start collecting your receipts, invoices, bills ect as the financial end of year 5th April 2020, after this date you have 9 months to prepare and submit your self assessment and pay any tax and ni due Its always best if you are employing the services of an accountant to do so as early as possible so they can advised you along the way
2019/2020 Personal Allowances were only £12,500 for income Tax and £8632 for National insurance class 4. Unless you were born before 1948.
2020/21 Personal Allowances are £12,500 for Income Tax(no change from the previous year) and £9500 For National Insurance class 4. Unless you were born before 1948.