I had this exact scan at exactly this time last year! I got this letter too and phoned them for advice re the jam thing, she said that I could have anything at all I usually eat as long as there was absolutely no fat or dairy or protein, I had some dry fried stirfry veg and a slice of dry burgen, I eat that anyway. But she said it wasn't compulsory to eat only if I felt I needed to. My appointment was in the afternoon so felt the need to eat something, also if I go to long I get a liver dump and higher numbers anyway so for me eating was the better option, In the big scheme of things slightly higher numbers for a couple of hours -does it really matter? black tea and coffee is also fine, take a snack to eat afterwards.
Good luck with your scan