British Airways

You can buy it a seat on this airline.

British Airways rules

Violins, violas and small wind instruments travel in the cabin if the case fits within 80 × 45 × 25 cm. It counts toward your hand baggage allowance.
Instruments between 80 and 140 cm are not allowed as hand baggage. They go in the hold or in an extra seat, if within 140 × 46 × 46 cm.
Guitars cannot be hand baggage.
Book the extra seat at least 48 hours ahead, by phone or travel agent. It must be a window seat next to you, in Economy or Premium Economy.
Hold instruments need a hard case. Over 23 kg may be charged; 45 kg is the maximum.

British Airways instrument policy

Violins, violas and small instruments in a case up to 80 × 45 × 25 cm are accepted as hand baggage.

https://www.britishairways.com/content/information/baggage-essentials/musical-instruments