Self-assessment tax returns for 218/19 may be submitted on paper, but the deadline of 31st October has already passed for paper returns. Subsequent 2018/19 tax returns must be filed online in order to avoid an automatic filing penalty of £100.
Therefore, you will now need to register for HM Revenue and Customs’ Online Services and obtain an Activation Code. Please note, it can take seven working days to receive your Activation Code, which means you must register by 21st January 2020 in order to obtain your Code in time for the filing deadline of 31st January 2020.
Alternatively, an accountant can file your tax return for you provided they use specific software - even if you haven’t completed the official form to authorise them to deal with your affairs. They’ll need your Unique Taxpayer Reference (UTR), but you don’t need to have registered for Online Services.