I went to an opportunities event back in July and my consultant told me that I would have to be 14st or less (i'm 5'9 so not sure on bmi for that). However, when at the event, I was told by someone else that as long as I am comfortable with my weight and still following SW then it isn't so much of an issue. The woman running the opportunities event said that SW does not penalise consultants who gain weight, just that you have to have to continue to be a member as well as doing your consultant role.
I used to have a very big consultant who banged on abut how she had lost 8 stone and then piled most of it back on, whilst then spending almost the whole of image therapy telling us all the crap she had eaten that week. In that case, I think she was a rubbish consultant and when I rejoined this latest time I looked for a local group that wasn't run by her. I mentioned her in passing to my current consultant who is area team leader and she said that in our area they were hang a massive shake up and retraining a load of consultants. I do think that even if a constant isnt necessarily skinny they can be good, they just need to believe in the plan and I think they should be following it, not saying all the stuff they eat off plan. That's just bad advertising for SW, really!