I agree with
@db89 ,s Glucojuice recommendation.
Kinda pricey in Boots, about £1.60 for a small bottle, so I tend to keep it for serious, hard drops when I've made a major miscalc.
It's pure dextrose monohydrate, and is just insanely fast, fast, fast when you need that sort of thing.
Tastes quite nice too and a dinky little bottle, so no probs just tucking it away in a pocket for
those full on emergencies.