how many hours i can work legally?

hi i work in sainsbury 20 hours a week then i applied for my extension and send my docs on diploma certificate that i am studying diploma which is undergraduate, the extension was successful they sent me the Card visa but they mentioned that i can only work 10 hrs a week during term time, then i changed my course and now i am in doing graduation, but sainsbury managment told me that i cant work 20 hours even if i get a letter saying i am a student of graduation and can legally work 20 hours a week, they say that i have to send my passport to home office stating i can work 20 hours legally, anyone can please tell me what should i do?
thanks

Conditions of your stay.