(I feel a bit intimidated following the famous Pucci, but never the less )

As often in life and more often with BA, the answer is " it depends". It depends on the staff present and your 'attitude'. Asking politely has gotten me far! Last year on a return from Canada, when an acquaintance did not turn up at his assigned 62A (I had grabbed 62K first) I asked if they could check the Passenger Manifest to see whether he (gave his name) was actually on board and perhaps upgraded to F. The CSD came to see me (was it you Pucci?) with the PM, verifying the name, and she told me he was upgraded to 1A and I could go an see him if I wanted to, so I did (but by the time I did he was already fast asleep).

I am sure that 'visits' from C to F are more common and more often tolerated than visits from Y or W to C.

On long hauls, I sometimes do try to stretch my legs by walking the complete circle (except F when seated in C) and only once have I been accosted about walking from C to Y and surprisingly never when returning to C… On BA 747s I am always seated on the upper deck and I have never been challenged about walking from upper to lower deck or vice versa…

It all depends… if needed, ask nicely, and if you belong there, act as if you do (without being arrogant of course)

(edited for 2 typos)