It may be helpful if you were to note the full analysis of the taxation of US dividends received by UK resident US citizens.
• The dividends are paid without any withholding tax as you are a US citizen.
• Under the treaty, you can claim a notional US 15% withholding tax in computing your UK tax.
• The US then needs to give you credit against US tax for the above UK tax. So you should include a claim for this in your US tax return.