I'm sorry I've insulted, you, BuiltForComfort. That wasn't my intention. I meant that as a general policy people should be hesitant to give out information to an unknown person on the internet which could help them to track a particular child down. Thanks to a distinctive uniform and helpful mumsnetters, a child sighted once, somewhere in London, could be traced quite easily. I cannot believe people are comfortable with this.
"If schools didn't want their pupils to be identified publicly they wouldn't ask them to wear uniform. And they wouldn't display their own crest on that uniform." You are right, of course. The fact schools are specifying distinctive uniforms doesn't make it a terribly bright idea with respect to pupils' safety.
The information about the school's uniform is in the public domain, obviously. So is the exact address of, say, "Jane Doe, aged 14, who lives somewhere in London." But that is also information I wouldn't give out freely to strangers. It's with good reason that pupils are advised never to post photos of themselves on the internet in which they are wearing distinctive school uniform.
FWIW, OP, I did (and still do) think it is very unlikely that your intentions are bad. If they were, you would have perceived your own question to be dodgy and would have gone to some lengths to sound plausible. You would have concocted some story in the first place to explain why you wanted to know. The fact that you didn't do so suggests to me that it never crossed your mind that asking about a school uniform might be suspect.
The reason I posted was to encourage people to think twice about giving out such information in future. Clearly I am barking up the wrong tree. Or maybe just barking.