But not the complete answer.
It can happen that the guage itself is defective. I have had this happen on the same aircraft type with the same result.
BA's spokesperson was correct, in that the aircraft would not have dispatched with passengers, had it been a known problem before departure. However once the problem arises in flight it becomes a case of how best to manage a difficult situation.
I am surprised the Daily Mail took an interest, as an outbound flight to the Cayman Islands probably wasn't carrying illegal immigrants, Princess Diana, nor were the effects likely to influence house prices.
It is not that the toilets become totally unusable, it is a case that the vacuum flush stops working. Obviously this limits the capacity to the bowl itself, and putting it delicately, the optional use of the facility.