they do that all the time. it's perfectly normal, if that possible.

They might be practising for something (I certainly wouldn't rule the next invasion out) but when I was in Cumbria in 1983 my eardrums nearly burst after a very low-flying RAF jet went over. In those distant days of course the Russians were supposedly coming, but the British were not about to invade countries just like that, mainly because there were two superpowers in the world at the time and one of them was not the United States. You just may be unfortunate enough to live near an RAF base and be on some kind of regular flight path.