See here on ba.com and here is an older thread too but seems to have good advice.

https://www.flyertalk.com/forum/brit...0not%20working.

CPAP machines

Medical clearance is not needed to travel with or use a CPAP Machine for sleep apnoea on board our aircraft. However, we recommend that you carry a letter from a medical professional stating why you need the machine for your journey.

You can carry your device in the cabin as an extra piece of hand-baggage without charge.

If you don’t need to use your CPAP machine in the cabin, and you want to check it into the hold, it will be permitted in addition to your free baggage allowance without charge.

If you need to use your CPAP machine on board, you will need a DC adapter to plug into the in-seat power supply where installed. We recommend always carrying a dry cell battery for your device should seat power not be available.

The maximum output of our in-seat power supply is 75 watts. If your machine needs a stronger output, you will need to bring a dry cell battery to power it.

https://www.britishairways.com/conte...-and-pregnancy