Find the canteen, not the fast food places (although how they can ban smoking and still rent out to burger king and Costa is beyond me), anyway I digress, there's usually a canteen and you can pick up a good on plan meal there, I recently got a breakfast, cut the fat off the bacon etc still synned it at about 3 syns because i didn't know how it was cooked etc, and they'll have fruit/salads.
Most hospitals have them to provide subsidised meals for staff xx
I prefer OUR Asda Cafe to OUR Tesco's but have been in other Tesco's which are very good.
However, back to the OP - if it's just for the day and you're that worried, then take something from home. > I have treatment every 3 weeks in our Chemo Unit & if my appointment is around lunchtime, I can choose from a brilliant range of sandwiches, fruit & yoghurt (there's crisps & biscuits too) that comes around on the trolley. > I've also taken bits with me & the lovely volunteers who keep us going with tea & coffee will nuke it for me in their micro's if I ask.