Frankie Hutchinson, Tours & Projects Manager of the London Symphony Orchestra, has persuaded British Airways to make clear on their website exactly which instruments passengers can carry as hand luggage.
Here is the new policy:
You can bring your musical instruments with you when you travel with us.
You can take small musical instruments in the cabin as part of (but not in addition to) your free hand baggage allowance, subject to the space available. We will make every effort to accommodate your violin or viola in its hard case in the cabin, even if the case is slightly larger than our maximum baggage size, as we know temperature and pressure can damage these instruments in the hold.
Larger musical instruments, such as guitars and cellos, can be carried in the hold or you can buy an extra seat to carry them with you in the cabin.
Please always transport musical instruments in a hard case. We cannot accept instruments, such as guitars, in soft cases as we don’t want them to get damaged.
And here is the website link.
Print this out and carry it with you when you fly BA.