In an article on anti-Americanism around the world, the BBC correspondent Justin Webb remembers attending Quaker peace rallies with his mother. He asks, however, whether the Quakers are fair in condemning US failings in areas such as arms control and world peace, while largely staying silent on worse offenses by others (BBC).
Personally, I think that British Quakers condemn the US more than others over violations of peace and human rights for two reasons:
(a) The US if a long-time friend and ally of Britain's, and the two countries have many cultural similarities, not least of which is their shared language. As a result, British Quakers (and other British protesters against American actions around the world) may feel a certain right to criticise the US more than other countries.
(b) The US sets out very high standards of freedom, justice, democracy and equality for itself and its citizens. Thus, it might sometimes be easier to criticise the US for failing to adhere to its own standards when dealing with foreigners (or even groups of its own citizens) than it would be to condemn other countries (or their leaderships) for failing to observe standards they don't even fully accept.