The nationality question and questions about the number of Brits as BA seems to crop up on a regular basis...BA's requirements are pretty clear:
"A valid passport (with minimum of 12 months before expiry date) allowing unrestricted worldwide travel
The unrestricted right to live and work in the UK".
Remember the unrestricted right to live and work in the UK can be gained through family ties/links, not just nationality (e.g being an EU national).
Given what we see on the Flight Deck every day I'd say (as previously stated) there's a wide spread of nationalities.