I'll have to dig out the official advice / FAQ for EC261 but I don't believe I am mistaken about non-EU --> EU --> non-EU itineraries not being covered by EC261.

As for UK261, I accept from advice upstream that the UK has roped arriving EU airlines into the compensation scheme.

I understand that the difference might be minor for BA board participants living in the UK, but it certainly isn't for others doing North America to India / Africa / etc. via the EU...