Foreign (US) pension income withhholdings

I am a UK resident citizen. In 2021 I received pension income subject to US 30% withholding by my provider despite my request for 0% under the Double Tax agreement. I understand I cannot claim FTCR and must reclaim the US tax paid if I can. However my Self Assessment form requires the disclosure of withheld tax and in so doing reduces my UK tax liability. The SA forms and notes are clear but I want to confirm the treatment. I find nothing to clarify in the notes or referenced manuals. Thank you

Hi,
 

If you cannot claim FTCR under the double taxation treaty, you will insert 0 as the amount of foreign tax paid . Thank you.