I think it is the preference of the Doctors and Surgeons working with you???
I did the 10 day milk and yoghurt diet i could have done it for 7 day but because i'm bigger they preferred me to do 10 days instead.
I had my Op through The Hospital Group and they referred me to Nuffield Hospital in Leeds with Dr Sarela (my surgeon) he is amazing, so is all the staff at that hospital!