I've actually had surprisingly good experience of the 350 in Y with its 9 across (although for full disclosure that wasn't on AY so not sure if pitch is smaller?). Much, much, much better than the 787 (except the JL 787), and in fact better than most aircrafts bar the UD 388 (which comes into play here but only one way) and UD Y on 747s on airlines which do that like AF. I actually would choose that if only for the much smoother transit experience (no security re-clearing, smallish terminal, AY non-Schengen lounge better than GC), lower risks of disruption at HEL compared to LHR, and avoiding unnecessary backtracking. If after 11 January, you'll also at least have free short haul water, tea, coffee and juice vs nothing at all for free on BA.

That said, the UD Y of the A380 on BA is good, especially window seats, and I find the 772/77W Y reasonably comfortable by Y standards too so not a bad option, but I would rate the AY one higher personally.