Nigel, the photo is from 1920. I think it was taken aboard Olympic at Southampton, I have no idea what month or date. There's also two photos taken in the Olympic's 3rd class dining room, and you can see the tables set up and menus on the tables, but I can't make out the date on the menu or any of the items listed.
I think it may have been impractical to keep the ships identical (in terms of layout). Every one of them offered something new and a little different. Olympic was the original ship and offered plenty of promenade space, whilst Titanic offered luxurious suites, Britannic too offered luxurious suites, as well as promenades and more luxury, some features such as the Cafe Parisien were not found on Britannic.
Regarding Olympic's promenade space, whilst in 1911 and 1912, you could walk all the way around A and B decks, after the 1913 refit, the extended restaurant and Cafe Parisien were installed. The rest of the B deck promenade was all there. In 1928 the forward modern suites were installed, however, you still had the length of the promenade on the port and starboard side along the staircase foyer and the B deck suites. So, there was still plenty of enclosed promenade space left.
Daniel.