Large companies inevitably require close employee liaison which can only happen by meeting people.

Complex projects particularly require huge amounts of liaison and cross-departmental liaison which results in even more regular travel.

I should imagine that BAE has figured out that, on most days, at least x people need to travel between the two offices. They have probably figured out that it's cheaper providing x seats (or slightly fewer than x seats) and using public transport for the numbers above x on that particular day.