Realising that it's a somewhat personal preference issue, what food is considered good for "eating on the go"?
Poached egg on toast seems less than optimal, for example.
I'm guessing you mean food that you can eat while riding? 
I go for mostly savoury stuff like Ginsters Sausage Rolls (although fat/salt content is often excessive) but mostly just sandwiches. The pasties or steak/curry slices are just too big and unwieldy to eat on the move unless you know you've got a long flat simple to navigate section coming up.
Stuff that is easy to hold in one hand whilst riding and small enough that you can eat it in two or three bites should you come to section of road where you need both hands on the bars (tricky junctions, a climb or a descent).
Pick the appropriate sandwich that doesn't have loose contents (BLT bad for this), has the right mix of carb/protein/fat/etc for you, and also won't make a mess of where you're stashing it. Chicken and stuffing works well for me but there are plenty of others.
Squash them down a bit after buying them, rip them into smaller chunks (for two or three bites) and bung them back into their packaging and stuff that in a jersey pocket or otherwise accessible bag. Try and spread eating everything out over the entire time between controls (this does take some getting used to, but at first just don't bolt the food down quickly in the first 5 minutes).
If it's raining it can be a lot trickier (soggy sandwiches aren't always fun) but then you can always fall back to quick stops under shelter along the way or just using the cafe controls (to eat their food, not your own food).
