The 'original' red kite reintroduction project was based around the delightfully named Christmas Common, near High Wycombe. I lived nearby for a year or so and can tell anyone who wants to know some top kite-viewing spots. When I left, about 7 years ago, I think they said that had about 150 breeding pairs. They are visible in numbers from the M40 north-west of High Wycombe pretty much any time!
They are much bigger than buzzards but make a very similar noise and the two species don't seem to cohabit much - so it's hard to compare what you are seeing. The definitive feature is the forked tail; a buzzard's tail is a convex fan whereas a red kite's is highly and angularly convex - really very forked.
I missed them enormously when I left the area. I love buzzards but there was something really special about the red kites.