Fox,
You need to make a clear distiction between sink and loo - I'm sure you do when washing your hands
In most areas there are two separate drainage systems - Storm and Foul. The storm drain is mainly rain water, which requires no processing prior to entry of a watercourse or reservior. Sink drains also often use this drain.
Foul drains take the discharge from the loo and are routed to the sewage works for immediate processing. It is these that are generally acceptable for the contents of your mop bucket.
In a previous thread, it became apparent that, though few water authorities would give a written answer to the question of the disposal of small amounts of trade effluent, the answers given throughout the country agreed on just the point that contaminating the storm drain was considered a cardinal sin.
Dave was the unfortunate one who did get a written answer, whereas I and some others were told that disposal on to soil, provided not adjacent to a lake or watercourse, was acceptable. The Environment Agency I contacted agreed with this.
One point that was consistant was that regardless of the content of the water, if it is generated by way of trade, then it is Trade Effuent.
John.