Don't forget queuing for the lifts. Due to the combination of mobility issues and being held up by a deliberately obstructive usher, you won't get to the lifts immediately.
I've followed Embarrassing's helpful link and found:
The four 15–17-person lifts (minimum 145cm wide x 148cm deep) have wide automatic doors (103cm), tactile buttons and voice announcements. All rise from level minus 1 to level 3 (stalls floor to gallery floor) and two also serve level minus 2 and level 4 (Pit floor and conference floor).
Unless otherwise stated, use these lifts to reach all areas of the Barbican. They open directly on to the foyers, except on the stalls floor (level minus 1), where the foyer is reached via sets of automatic doors separated by a ramped crossing.
Sounds good, but if you look at that in more detail, that means they have a bank of 4 lifts, 2 of which serve five floors (unless they number the ground floor as floor 1, in which case it will be four floors) and the other 2 of which serve seven floors (unless it's actually six).
At this point, anyone who has experienced being dependent on lifts in time-sensitive situations will be grimacing.
If you haven't been, what that means in practice is that at peak times, like the interval, it is just like peak hour traffic on the roads. If you get caught behind everyone because you didn't get out there when the lifts were all free for the first circuit you'll be waiting for one lift to drop off all its passengers and come back to your floor.
Then you have to queue in the toilets, which have reduced capacity now because there are fewer of them.
And when you've been to the toilet, you'll have to repeat it in reverse and recall the lift to the toilet floor, because of all the people who did get to the lobby first originally, who got to the toilets and finished before you, and took the lift back!