This subject pops up frequently and there are lots of existing threads on it. Crews have confirmed that on ba shorthaul, there is no blanket rule to limit people to using the toilet in their cabin or prevent them from crossing the curtain unlike us airlines.

crew logically use discretion and usually do it well. If you have an a319 cabin with C going all the way to row 10 and they are serving the meals, crew will logically ask y pax to go to the back. If the cabin goes to row 4 and the c service is complete while trolley is blocking the corridors in y, they will logically tell y pax to go to the front.