GOV.UK
Double Taxation Relief Manual: Guidance by country: United States of America: charities and superannuation funds
Old Agreement to 2003
Since a United Kingdom charity or superannuation fund is exempt from United Kingdom Tax on its income, it is strictly not a resident of the United Kingdom as defined in Article 4(1)(a)(i). Nevertheless, it is agreed by both the United Kingdom and the United States authorities that a charity or superannuation fund which is otherwise resident in the United Kingdom is to be regarded as a resident of the United Kingdom under the agreement and therefore entitled to the benefits of the agreement.
New Agreement from 2003
Pension schemes, employee benefits schemes etc, charities and a qualified government entity are now specifically defined as residents of the UK in Article 4(3)(a), (b), (c) and (d) respectively.