None of the bread at Subway has enough fibre to be used as a HEB, although I have used my HEB in the past if I've not had enough syns but I know it's not to plan. Salads in Subway are great though.
I have subway salads all the time because they're free!! If I feel that I want bread then I have my own wholemeal roll as my HEXB with it. You then find that you don't need the whole salad and can save half to have as a side on evening meal it for lunch the following day! X