So yes it is indeed a legal requirement for a toilet with access for the disabled to be provided.

One should keep in mind that disabled toilets are toilets that have been provided that incorporate certain modifications compared to the usual toilet; modifications that allow for easier and safer access for disabled people. So they are toilets that provide appropriate access for disabled people; they are not however toilets designated solely to be used by disabled people, anyone can use them!

In places of work the number of toilets provided will be aligned with guidelines that stipulate the ratio of toilets to people. One toilet per ten people for example. So a company has to provided enough toilets to cover the number of workers and in some cases the disabled toilets will need to be included in the number in order to meet requirements, especially in smaller companies with more limited resources. Companies are not in the business of providing resources that are never used!

The fact that there are no disabled people in the office means that there's more able bodied people and therefore it may be a legal requirement that the disabled toilet is available so as to provide the correct ratio. If there were a number of disabled people in the building then I could see an argument for making the disabled toilet available to them only (as long as there were enough other toilets to cover the remaining workforce), thus keeping them available at all times for the increased likelihood that a disabled person may need to use it. However having no disabled people in the building means that there is actually less need to give consideration to keeping that toilet available at all times and therefore more reason for able bodied people to use it.

I guess the cleaners concerned didn't take all that into account though!