So I would presume from some comments that those who are consultants are also excluded from using the "facilities" of the organisations employing them, and that some are not?
I'm not going to take sides, just to state my opinion & what I do as a transient worker.
I use the customers facilities, if they then try to stop me, I simply pack up and leave, never to return.
I generally use the facilities immediately upon arrival.
I have never been refused.
If they try to stop me from leaving then I would be looking at accusing them of false imprisonment.
Whether I could, would get away with it, or not, I would do that.
I see it as an insult and a breach of basic human rights to be refused use of toilet facilities.
As far as providing other amenities, I would expect use of vending machines, if present, use of a canteen area to consume food & drinks, or another place suitable, however, I would only be looking for showers in an emergency scenario if they were present on site.
Mind, some of the places I have been, I would not want to use the facilities a second time, not being able to wash after using them is a bit of an issue for me!
At the time, I was also engaged by that client to undertake practical mechanical engineering works on machinery, which resulted in getting dirty, and no where to wash, even though the official agreement with the customer was use of their facilities, which we had, just that the facilities were not up to scratch.
This became a common theme with many clients of the same type that we were dealing with at the time < 10 years ago, so we then arranged to carry washing materials etc. only relying on the client to provide running water & toilet facilities.
That contract didn't last long, they saw the facilities as OK, we didn't, we didn't try to win the contract when it came up for renewal and they wanted a cost down!