I have been doing some research into this, so that I can add (eventually) seat width info to the https://www.flyertalk.com/forum/brit...-products.html thread.

Here is the info collected so far. Bed mode measurements take into account retractable arm rests.

Aer Lingus
A332, A333, B752: 19.8" wide (19.8" bed mode)

American Airlines
B763: 19.8" wide (19.8" bed mode)
B772, B788 (Concept D): 20.8" wide
B772, B789 (Super Diamond): 21" wide
B773: 20.5" wide (25" bed mode)
A332, A333: 20.5" wide

British Airways
B744, B772, B773, B788, B789, A380: 20" wide (25" bed mode)
A321: 19.8" wide (19.8" bed mode)
A318: 20" wide (21.5" bed mode)

Cathay Pacific
A333, B772, B773: 20.5" wide (25" bed mode)
A350: 20.2" wide

Finnair
A332, A333: 19.8" wide (19.8" bed mode)
A359: 21" wide

Iberia
A332, A333, A346: 21" wide (21" bed mode)