You have said that you are only putting a wc and shower in, if that is the case the need for a "wet room" type of environment is not really required. I had a quick look on your heated floor suppliers website, they offer a library of installation videos, I would use this as your first port of call. As for tiling, I would not put a coat of PVA down without checking the instructions on your tile adhesive, some adhesives can have an adverse reaction to PVA and crystalise over time causing tiles to lift/blow.