Double Taxation Relief Manual: Guidance by country: United States of America: Relief from US tax: Forms
Claims by a resident of the United Kingdom for relief from United States withholding tax under the agreement are to be made on Forms in the W8 series;
These forms are downloadable from the IRS web-site as follows:
The form, which does not require certification by an Inspector, is sent by the claimant to the person in the United States who is required to withhold tax from the payments of the income in question under United States domestic law. The tax to be withheld from dividends will normally be at the rate specified in the agreement if the recipient has an address in the United Kingdom. Where United States tax has been withheld at a rate in excess of the rate specified in the agreement, the United Kingdom resident recipient should file a United States tax return in order to obtain a refund.