If I were a Muslim I would be unhappy about ham sandwiches, Ramadan or not - the prison service should know better than that.
Out of interest, where did you find out that all this Tesco meat is halal? It doesn't surprise me at all and it doesn't bother me one bit, halal meat is very tasty - but I would be surprised if there were enough Islamic slaughterhouses about to satisfy the supply requirements of an organisation as massive as Tesco's. We bought halal chicken for our wedding, as we had several Muslim guests, and my butcher didn't even know that he was buying halal all the time, he just bought it from a warehouse. I wouldn't know if I was buying meat that is Kosher either, and it doesn't bother me one bit.