Red kites were once a common in England, but they were classified as vermin and hunted to extinction in the nineteenth century.
However, in 1989 the RSPB and Natural England (or what ever they were called back then) started a series of reintroduction of red kites from spain into the Chilterns and have become one of the success stories of reintroduction programmes - in fact birds from the Chilterns are now being caught and released in the east midlands and yorkshire.
There was a time when they most likely to be seen near Henley or Watlington, however they are becoming more common and can frequently be seen over Abingdon, as seen in this photo, taken from my back garden.
Although they're raptors they mainly eat carrion (although they do catch voles and other small mammals).
Southern England Kite Group: http://www.sekg.org.uk/