The caffeine in Buckfast is a replacement for the Coca alkaloids that were originally present in it, from Coca leaf. Eventually laws changed and they had to find a substitute in the form of caffeine, from yerba mate. You can buy mate tea here, rooibos tea, it's caffeinated herb from south America. This would probably be your best source of the caffeine if your wanting to stay close to the original.
Makes you wonder what Paisley would be like on a Saturday night if they were still making it with the original recipe :shock:
DC