I was focusing on BA. Extending to other airlines - many, mostly depending on aircraft type rather than airline choice (is there any airline out there with absolutely no row 1 fleet-wide?).

This seems to be more common nowadays with staggered configurations, etc, such as certain IB aircraft that go 01C/02A/03C etc.

"Fun fact", the A321 midhaul fleet was initially supposed to be in a B-DF/AC-E configuration rather than A-DF/AC-F. Not sure whether they ever operated as such and only subsequently got renumbered - or should that be relettered?

Edit - just saw Deltus' post! There's your answer