The law says that you must consider alternatives. If a window cleaner then considers that to clean the windows a ladder is considered the best means then they've complied with the WAHL's. The wording is 'consider an alternative' for short duration work. Not 'must use'. If, when working for a company, the company consider the risk using a ladder is too great, then that has to be followed. For private property, the householder is not expected to know what is safe and what is not, so the onus is on the window cleaner and a private house holder is not liable for any accidents because of that fact unless the risk was a known fact before hand to the householder.
Provided the height is under 9m and there is some type of safety device for the ladder even if it's rubber feet on the ladder, then that is ok. ( having said that, I'm not certain if it's over 6m on an unsecured ladder)
Provided a risk assessment has been done, which for a specific private property does not have to be written, a written generic risk assessment is good enough, and provided the person undertaking the work has an understanding of the risks involved, then the WAHL's have been complied with.
As Dave says above, ladders are not banned and the thing about choosing the best method, the best method would to not do them by any method available. Then there is zero risk. Risk is an accepted part of working. Reducing the risk, and thus reducing accidents, is what the WAHL's are about.
Cheers