Self assessment form cwf1

Filled in a self assessment form to register first time for my son. Address on line said send to bx9 1an . Is this correct? Needs a UTR code . Will the letter get received and processed. I sent it signed for

Hi,
The postcode you have referenced is the correct address for form CWF1 to be sent in order to register for Self Assessment and Class 2 National Insurance.
The guidance on how to register for Self Assessment can be found here:  
Check how to register for Self Assessment
Thank you.