You can take tablet / powdered sports drinks along if you need as they may just be supplying water. It's also chaper than buying them pre-made. I use the Zero ones (just for salts). I used to use the powdered SIS ones but they taste a bit weird and leave a residue. Zero orange and cherry flavour ones also taste just like creamola foam which is a plus :)
Constant eating of small quantities works best for me, as you need to have the stuff in you and being digested and sent to your blood stream well before you begin to fade. If you proper hit the wall, the body can be very good at diverting its resources to vital systems, which often means that your ability to eat, digest and absorb food is slowed.
Bananas, jelly sweets, tablet, Kendal mint cake, tea with lots of sugar are all good as sources of simple sugars. If you compare the energy content and sugar composition of gels to Jelly Babies there's very little in it. And they taste rotten, just throw a fistful of JBs in your mouth for the same effect.
Heavier foods (like your Sunday roast suggestion!) don't work for me, that's not to say they won't work for you, but for most they don't.
A good meal including carbs night before, a porridgey/meusli-ish type breakfast with lots of slow-release carbs and filling yourself full of fluid until you keep peeing is a good way to set yourself off (in my experience)