My "London face"

I was introduced to a fascinating idea the other day from a colleague who was visiting London for the day. He had at one time worked in London and then had moved to the West Country. Whilst we waited for the meeting to commence he told me that he hadn't been in London very long that day and he had found himself re-adopting his "London face". I was intrigued by this and asked what such a face looked like. He told me it was essentially a blank stare and he proceeded to demonstrate it to me.
It was as he said blank. In some ways similar to the blank expressionless stare that some people with Parkinson's disease have. Devoid of expression and at the same time a bit threatening.
We discussed what the purpose of such a stare was. He considered it was to avoid contact, in particular to avoid interactions with people who are trying to sell or beg. I was familiar with the tricky issue of eye contact on the Tube and how that took some managing on a busy trip, but this really interested me. Was it a way of keeping to oneself, or a means of keeping people away.
A look that says "don't bother me".
As I was making my way to the Tube at Oxford Circus I noticed that all three Big Issue sellers I had passed didn't offer me their magazine.
I became worried, was I wearing my very own "London face"?