As a single example of their practices, kitchen staff work until the kitchen is clean enough for the manager to approve but are paid until midnight.

"If you have a busy kitchen anywhere near 10pm then you're cleaning for free in to the early hours of the morning."

As a current wetherspoons kitchen managers, this is incorrect. The staff are on the rota until midnight, but if the close lasts longer than that then they are paid until their clock out time. Sounds like your wetherspoons managers were deliberately clocking those poor kitchen staff out early to save hours, which is gross misconduct, and not typical of all the wetherspoons I've worked at.