It looks like our (UK) Herring Gull (Larus argentatus), and seems to exhibit similar defensive behaviour! I didn't notice a nest though?
The behaviour is pretty clear anyway. They will swoop at an intruder with or without legs down. They will attack humans if their nest site is threatened, sometimes striking them with enough force to draw blood or render unconcious (Birds of the Western Palearctic , Oxford, 1985). They will also call to attract nearby gulls and mob the invader, and defaecate towards their target, so watch out when ... .... .... ... fan!!
I have a picture somewhere from my rock-climbing days of a fibreglass helmet with a fist sized hole in it made by a herring gull!! But I think you were far enough away not to present a serious threat to whatever it was defending.
Happy flying from the UK.