Han,
When you log in to BA recruitment and click 'Heathrow - The Rivers, Cranebank' it advises that you may use the BA1
A British Airways courtesy bus, the BA1, runs to and from Hatton Cross Underground Station every twenty minutes. You should aim to be at Hatton Cross at least 40 minutes before your appointment. You must show the bus driver a copy of your assessment confirmation email to be allowed to travel on British Airways courtesy buses.
Regards