UK freelancer and certificate of tax residence

I have completed some freelance work for a Portugal based company and they advised I will need a certificate of tax residence for them to pay me. When I fill in the form it asks how much I've been paid, so I assume the form is valid for money already earned. Looking into it, it seems gov.uk should be able to issue a confirmation of a UK tax paying resident instead. However, the company waiting to pay me seems adamant I need the certificate before being paid and I can't see how to get a confirmation letter. Can anyone advise how I proceed to get paid?

Hi,

A certificate can only be issued when foreign income has been received other wise it is a letter of confirmation that will be issued.

Thank you.

Thank you for your response. I have not been able to find an option to get a letter of confirmation, could you advise how I get this?

Hi Edb,
You should request a certificate of residence, by completing the online form at How to apply for a certificate of residence to claim tax relief abroad.  
If you have no foreign income to declare, but require a letter of confirmation of residence, because you are required to provide evidence to an overseas authority
that you are resident in the UK for tax purposes, to meet a requirement of an overseas countries local law, then we will issue a Letter of Confirmation of Residence.  
If the request is solely for proof that you are living in the UK and is not for tax purposes, a Letter of Confirmation is not appropriate. In these circumstances, you will
need to contact the overseas authorities to determine what they will accept.
Thank you.