Freedom to Photograph
There are still some members of the public that believe it is illegal to take pictures in a public place, this is not so. However, caution must be exercised where children or sensitive establishments are involved.
In order to minimise any disappointment, you may like to carry a copy of this Metropolitan Police document with you at all times.
Metroplitan Police - Photography Advice
The railways similarly, have their own document,
National Rail - Guidelines for Rail Enthusiasts
Last Updated, 19/06/16