FAA safety regulations, PART 382 NONDISCRIMINATION ON THE BASIS OF DISABILITY IN AIR TRAVEL
In particular sections 382.41 (c) / (d)http://tinyurl.com/ojenq
"Response from the FAA Re: CPAP as carry-on (long)
Section 382.41(c) of our rule on air travel by passengers with disabilities (they attached a copy) states "Carriers shall permit qualified individuals with a disability to stow canes and other assistive devices on board the aircraft in close proximity to their seats, consistent with the requirements of FAA safety regulations for carry-on items." This provision is not limited to devices that are to be used during the flight, i.e. it applies to all assistive devices used by passengers with disabilities even if the devices are not used during the flight. Part of the rationale for this provision is to minimize the likelihood of damage to assistive devices.
The FAA safety regulations referenced in the above provision basically require that the device be stowed under a seat or in an overhead bin, or secured elsewhere in the cabin by the crew. In addition, if the device contains a spillable (e.g., wet cell) battery that is detachable, the airline is free to detach and package the battery and carry it in the baggage compartment of the aircraft.
As you noted, section 382.41(d) of our rule states "Carriers shall not, in implementing their carry-on baggage policies, count toward a limit on carry-on items any assistive device brought into the cabin by a qualified individual with a disability."
All U.S. airlines are required to make available upon request a Complaint Resolution Official when questions or disputes arise over the application of our rule on air travel by passengers with disabilitites. CRO's have received special training about this rule. Airlines are free to make a CRO available by phone. See section 382.65 of the attached rule. You might also want to carry a copy of the attached rule and this reply with you. And it might be wise to notify the airline in advance that you will be bringing the device and a carry-on bag into the cabin, and to ask them to insert a notation in your reservation record (your "PNR") with the employee's name and position stating that this is approved, so that any agent who might question the machine when you check in can be referred to that PNR entry.
This rule applies to all operations worldwide of U.S.-based airlines (except with respect to fixed airport facilities outside the U.S.). The rule does not apply to foreign air carriers. I hope this information is helpful."http://www.sleepnet.com/apnea38/messages/539.html
This applies to all US carriers operating in the UK, but I'm not sure about regulations for British and other airlines - try under CAA or EASA?