Yes, it is essential. It is your responsibility to register with HMRC as self-employed. Registering with HMRC is a straightforward process and all the information required, including help lines, is on the HMRC website. You can also look at Register for Self Assessment: Register if you’re self-employed – GOV.UK (www.gov.uk)
You should allow 10 working days for the registration process (21 days if applying from abroad).
HMRC will not accept a registration for self-employment more than 28 days before the work commences.