When the US requested the forced expulsion of the Chagossians, the Queen could act unilaterally since she is still the direct ruler of the British Indian Ocean Territory.
According to Wikipedia, there is a commissioner with the Foreign Office that acts as administrator, but that commissioner is directly appointed by the Queen [2].
1) http://en.wikipedia.org/wiki/Stealing_a_Nation
2) http://en.wikipedia.org/wiki/Government_of_the_British_India...
While this may be technically true, the monarch's powers (the royal prerogatives[0]) are almost exclusively exercised on the advice of the prime minister/cabinet.
In other words, while the power officially rests with the Queen, the UK government is the one to wield it in practice.