You seriously didn't know about own brand energy drinks? All supermarkets have them, asda, tescos, sainsburys, waitrose, M&S even i think.
Go to lidl and get the stuff there for about 25p a can, cheap and it does the job.
Or buy some pure caffiene powder and a measure (it's about £5 for 50g's, the LD50 is 2g so you can imagine how far it goes) and put it in squash/ water. A red bull contains about 80mg of caffiene, which is not a lot in pure powder form so you'll want some micro measuring scales.