I'm assuming that you are a bit anxious about this - its making you sound more than a little entitled and unreasonable though!
You may well usually eat your evening meal at a certain time, but that doesn't mean you can't eat supper shortly before the time the hospital has set your fasting time. It's a simple and sensible way to reduce hunger if you are waiting longer than expected.
You obviously want the procedure done or you wouldn't have got this far. Don't mess people around (and put yourself back to square one) by walking out of there is a delay. Bring a book/podcast etc and plan a lovely meal for afterwards. Hope the op goes well!