I would say sri lankan is the best option out of the available lot. have flown them in many occations and the standerd of service is second to none.
I also read in a flight magazine recently(dont remember the name) that Srilankan air lines have won the "friendliest cabin crew in the world"award!
There are other options too on the said route such as EK-QR-GF-SV...,however I find those services are either not up to the mark or cabin crew are very robotic or simply an exageration of advertising/publicity.
Also where the entertainment is concerned,Sri lankan has one of the best systems equiped onboard there air crafts and is 100 per cent identical to what EK has.
Last point is,Sri lankan has direct/nonstop LHR-CMB-LHR services and is a time saver.
Hong Kong Layover.