Thank you for your question.
As your rental income is in excess of £2,500, you need to file a tax return. If you have already not registered for self assessment, you can do it here
As you have said you have no other income than rental income. This would be covered by your tax free personal allowance.
You may be fined fixed penalty for late filing of tax returns.
If you don't have a UTR number, you won't be able to file your tax return online. Once HMRC issues you with a UTR number, you will have 3 months to file your tax return for 2018-19 as we are too close to last date for filing online (31 Jan 2020).
You should start the process immediately. You should be able to file your tax return yourself. If you ger stuck, you can come back here and ask for help.
My aim is to give you a professional service. I hope this is helpful and answers your question.
Please remember to rate my service by selecting the 5 stars at the top of the screen before you leave JA today. If you need more assistance, please use the reply box below and let me know. It has been my pleasure to assist you with your question.