I've not been able to find much in my research online. What I've learned (mostly here) is that the Britannic was built but not yet fitted out with her luxurious interior fittings prior to the beginning of the war, and that she never got to serve as a normal ocean liner before being converted to a hospital ship. What would have been changed in the interior, and what would have been more or less identical to the Olympic and Titanic?
For instance, would the fancy wooden panelling and linoleum flooring in areas like the grand staircase and dining and smoking rooms have been kept (if they were already installed?) or swapped out for something more plain? Would the gymnasium and swimming pool have stayed or been converted into more rooms?
Sorry if this isn't the right sort of question for this sub. There doesn't seem to be a whole lot of info that I can find so I understand if no one has any solid answers.
Unfortunately the most detailed readings that I've encountered give the ever-ambiguous statements along the lines of "the dining room was changed to accommodate a ward." It's not that helpful because it could mean anything from removing all the panelling and fixtures down to the bare metal to just removing the furniture and leaving everything else.
This question was asked on Encyclopedia Titanica (which I highly recommend you browse around in) and the general tone of the conversations was they didn't know.
Going on just speculation here, but it would make sense that the White Star Line wouldn't want to send Britannic filled with all her expensive furnishings to be used as a hospital in a war zone where she would rarely be maintained.
Perhaps a good way to examine this would be to look at how Olympic was changed for wartime (though she was a troop transport) and how far along Britannic was when she was requisitioned.
I wish I could be of more use to both you and myself.
Oh. Forgot something. Again.
There is an organ in Switzerland that was apparently built for Britannic. I'm going to look into it because it was marked "Britanik" which was not the spelling H&W would have used. More importantly, it didn't use 433 which was the hull number which is how H&W organized the materials for each ship.
My understanding was that instead of having the Boat Deck to A-Deck stairwell there would be this large organ in a large open space. I'm not sure where the stairs would have been instead because on Olympic and Titanic that was the best way for passengers to get from the Boat Deck to the rest of the ship. The plans of Britannic I've seen seem to have the stairs intact.