I Flew UK->US and back, UK->Dublin and back, UK->Greece, and half a dozen others I’d forgotten during the social transition but before I had a passport or driving license in the appropriate name/gender.

Had one border guard in Greece be a little awkward and one guy before boarding a BA flight to the US at Heathrow say I didn’t have the right passport (who was promptly slapped down by his supervisor).

But generally zero issues.