Porridge for (part of) breakfast first (which I've finally trained myself to like, after being force fed it as a toddler and growing up hating it). It has steady energy release which lasts a long time.
Jelly babies, in a small screw topped container for in the cave. You can dispense them into your gob without the remainder getting wet. You can also "drip feed" the supply of energy, rather than having bigger snacks less frequently.
(I started using these for caving as a result of using them on fell runs, to maintain energy levels.)
Hasn't this topic been discussed before on here? There may be some useful info already available.
Hydration is more important than fuel! I used to take saus rolls in a tin, but I've either toughened up or become more efficient as although I still pack a few choc bars (dark choc bounty for preference), I don't always eat them.
Capri-sun do screw top drink packages (not the silly straw cartons that they also do) which are squishable (gentle) fit into tacklebags and re-sealable. I often take 1-2 with me.
I used to make a "brick" of sweet cous cous with honey dried fruit and pine nuts. If you're doing Cuthbert's on Mendip leave a mars bar at the bottom of the entrance rift, you'll need it on the way out. Those squishy packs of lucoz drink are handy. Individual Malt loaves are cave proof - mind you pack them in a Darren drum though
I used to have an emergency mars bar in the headband of my helmet, along with a space blanket