Overseas income not included in tax code

Good evening. I receive payment from an online company in the USA for some consultancy work I do and include this in my self assessment. I have however been informed that I cannot be taxed on this via my tax code and will need to pay a lump sum based on the tax calculated once my self assessment has been submitted. Why can this not be included in my tax code? Thanks

Hi,
The tax payable against self employment income, whether arising in the UK or overseas, cannot be collected automatically and tax codes deductions that can be made are those that are expressly permitted by statute.  EIM31600 provides details of deductions that are permitted by legislation.
EIM31611 - Deductions from general earnings: where to find instructions
Self assessment has two payments on account, that allow the payment of tax in advance, arising from the net profits from self employment.
Thank you.