Surprisingly, whilst what he was doing is on the limits of bad taste, it is not illegal.
On a photo course a while back, the chap leading the course explained that if you're in a public place then you have no right of privacy, i.e. to not be photographed.
Sadly, the copper probably knew this too, but it's really despicable behaviour and totally voyeuristic.
I blame the 24 hour news broadcasters and the tabloid newspapers.
Well done for not giving him a slap.