There is of course the plain old fish & chip shop in Long Lane - opposite the Valentine pub - they do cook a nice piece of skate while you wait and you can also phone and order. It's take away only though.
I've missed the boat concerning your friends' visit but if you are ever looking for something more local I would recommend the Old Operating Theatre and Garret. Its at the junction of Borough high St and St Thomas St, (opposite the Bunch of Grapes pub). I'd recommend it to anyone and particularly so if you call up before-hand and arrange to visit when they are giving a presentation. When I went we were shown Victorian surgical practises related to the removal of gall stones!