FYI,
After I told TIAA-CREF that Fidelity do 0% withholding on IRA payments made to US citizens resident in the UK they sent me an email assuring me that my feedback and questions were very important to them and that they will review their withholding policy. I don't expect any change, but I get the impression that TIAA-CREF had never considered the tax treaty as it was never mentioned in the explanation of their policy, whereas, the treaty was front and center in Fidelity's explanation.
I have also asked Fidelity to give a bit more detail of the reasoning behind their 0% withholding policy, specifically why the 0% applies to a US citizen when the saving clause usually prevents US citizens from claiming treaty exemptions. I hope to get something that mentions Article 24.