Here is what Virgin Atlantic says about carry on baggage and musical instruments - virgin-atlantic.com/us/…hand-baggage.html
"Maximum size - 23 x 36 x 56 cm (that’s approx. 9 x 14 x 22 inches)"
If you’re planning on bringing a musical instrument with you, bear in mind it may not fit within your hand baggage allowance.
If it exceeds the hand baggage allowance measurements, you’ll need to check it in (and pay an additional baggage fee, if it takes you over your check in allowance). However, rather than checking it in on the spot we’ll still ask you to carry it through to the airport security checkpoint (subject to relevant Airport Security approval). As instruments are especially fragile, this ensures safer carriage. It will be taken at the departure gate for loading into the hold of the aircraft, so please make sure it’s suitably protected when you pack it.
Alternatively, you can carry a maximum of two instruments onboard if you book a seat for each one. Each instrument cannot exceed 75kg (165 lbs) and its centre of mass should be no more than 30cm (12 inches) above the top of the seat cushion.
For any questions about bringing a large musical instrument, please contact us in advance of your flight.
Little Red (UK domestic) flights - Musical instruments are not permitted as hand baggage on Little Red (UK domestic) flights and must be checked in as check in baggage."
All pretty clear and straightforward, and available for reading before booking.
Edited: 8:59 pm, June 25, 2013