All BA planes I know, if they have mains outlets, at least have ones capable of accepting a UK 13A plug. Normally they are capable of accepting others (e.g. US) too. From vague memory the A350 has these.

However, I would bring a power bank anyway. I have had too many experiences of the power socket either not working at all, "working" but switching off when it has any load on it (like a laptop charger) or being too weak to charge a laptop. If you actually need to use your laptop, bring a powerbank. I have had slightly better experiences with the USB-A ports on planes being able to charge a phone, but not much better.

FWIW I have an Omnicharge Omni 20c+ (20000 mAh, 100 W) which I quite like and is plane compatible. It's only issue is that I often forget to pack it.