His channel is "
Guerilla Warfare Piano" suggesting that he doesn't seek prior permission.
While the other group of people present themselves terribly, for a pianist the guy has a lot of videos on his channel of him interacting with security. Here's one of him at Kings Cross, he moved the piano and didn't want to move it back when asked to by station staff. Subsequently, he was escorted out by police though his videos incorrectly have him as being "arrested".
They were apparently only recording a small segment for a greeting, St Pancras allows filming but asks that you apply if it is for commercial purposes. It allows them to ensure that the piano is still shared for public use, and security being aware of filming helps them manage situations like this.
The Elton John Piano is a lot more famous. Trying to get photos of the Kings Cross piano is quite difficult as Google Images gets photos of the Elton John piano over the road.