For purposes of photography, is a bus a public place?

(England) I was on a bus owned by Carousel / Thames Travel this morning. The bus was serving a publicly-accessible bus route through a number of towns and villages. A member of the public was seated on the bus taking photographs of passengers as they were paying their fare.

Does the bus fall under public-space, or private-space rules?

I understand that if it is a public space he is within his rights to take photographs, however before I go to the bus company to complain I'd like to have a better understanding of where the lines are drawn than I can get from a quick google and read of articles for the layman.

Thank yu!