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*

And then goes on to say,

Taking photographs of a person in a public place would not normally be regarded as an invasion of privacy ...

The Photographers rights in the UK PDF also says,

Taking photographs is unlikely to amount to a breach of the peace or to conduct likely to cause a breach of the peace ...

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Martin