My partner has used his CPAP machine several times on BA, but always in F or J - there are some good seats in J on the upper deck of 747s that are perfect for resting the machine. He used the inseat power, always asks the crew before using it, and has never had a problem.
BA's website is very clear and in stark contrast to backdoc's shocking Virgin experience states: "Medical clearance is not required for the carriage or use of CPAP machines (used in the treatment of sleep apnoea) as fitness to travel will not be in doubt." Laptop power is rated at 75w and as long as your device needs less than that you are fine, although BA do not guarantee that they will be in service and suggest you use a battery-powered machine if necessary.