You do not have to register as a Sole Trader until the 5th of October in your business’ second tax year. A tax year runs from 6 April to 5 April the following year.
While you can trade whilst you wait for your Unique Taxpayer Reference number, HMRC suggests that you should register with them as soon as you become self-employed.
If you are working in the Construction Industry (CIS) you can work without a UTR, but you will pay more tax. You will pay 30% tax without a UTR & CIS and this is reduced to 20% once you are registered.