The BA B777 even when manual flying, use of the A/T is mandatory, so you can't win.
How the heck does that work in the event of unreliable airspeed ? A/T carries on setting thrust based on known bad air data ?!
No need to get worked up about A vs. B or not being able to disconnect the Autothrottle. For normal ops the BA 777 SOP is indeed to use the Autothrottle at all times when it's available and functioning correctly,
however in the event of unreliable airspeed indications one of the several initial actions is to disengage the Autothrottle and use "manual" power.