I wouldn't even know what a "starter bay" or "extension bay" was. I also think there are WAY too many options.
If you are involved in the hospitality industry you would think you'd understand what these terms mean, having said that would those terms be specific to a UK audience? For example, I'm in Australia (I have a fast food business background from a previous life) and we would just call them shelving or store room shelving. The starter bay, we'd call it a "return". I ask this question because I noticed that your client does take orders from outside mainland UK.
You are right about having too many options. That is a lot of stock that the client is carrying.