I have looked for answers but cannot find anything conclusive.

I need to take a flight from Heathrow to Dubai, on to Columbo in Sri Lanka and then back to Heathrow.

I’m hoping I can do this on one booking but the carry on looks to be a problem. I have a standard bag that squeezes into the BA carry on limits but is too big for Sri Lankan airways. However it appears that if the connecting flight is part of the same booking then the larger carry on limit applies.

Does anyone have any experience of this situation?