I would like to think everyone can stay in their cabin but it is not practical. If there is a trolley blocking the aisle using the toilet at front is the only alternative unless you can hold it in. If you are travelling with a young child who announces they need the toilet when they are completely desperate you cannot wait in a queue or wait for the trolley to pass, you need to go.
Similarly, I found on a non BA 788 when I was sitting just behind the curtain after the second door, the nearest toilet was on the other side of the curtain but the toilet I was meant to use was at the end of the cabin. When there are 20 people all wanting the toilet after a dodgy meal service there cannot be rules on using the toilet, especially when the nearest toilet is empty.