30th September following end of tax year if filing paper version and 31st January for electronic version.
The UK tax year runs from 06 April to 05 April.
You have almost 10 months to compile the relevant information for your tax return if filed electronically before the deadline of 31st January
The filing deadline is also the payment deadline for tax due. Automatic financial penalties apply hereafter, with interest and further penalties accruing the longer it remains unpaid.