From the Photographers rights in the UK:*
It is not always easy for a photographer to know whether taking or publishing a photograph might amount to an invasion of privacy

*I've always thought that laws were written as a guide that should be adapted for and in the confines of any given situation. The above line does strike me as odd however.
I guess for some, simply reversing the standpoint for any given situation, doesn't always work. This is dependant upon the character of such a person. But one should ask, whether the law is on your side or not, is it the moral thing to do.....
Should we only obey laws that suit us? Well sometimes, as Onslow pointed out, the law is an ass...well then all we can do is rely on our own moral understanding of any given situation.
What i'm saying is, if your own morality excepts that it is alright to photograph anyone in this situation, well who am I to ague.