The pattern
YouTuber with around 40% of audience in the US, AdSense around £45k a year, W-8BEN never filed. Google was withholding 30% on the US-viewer share.
What gets done
W-8BEN filed correctly inside AdSense. US withholding rate dropped from 30% to 0% under the UK-US treaty. Direct annual saving on US-share withholding alone in the £5-7k range at that AdSense level. Foreign tax credit claimed on Self Assessment for prior-year residual withholding.