Our NCT MW is also an advocate of avoiding caffiene during labour, as it can increase anxiety levels. The opposite of what you're trying to achieve really.
As I don't drink tea or coffee with caffiene (I've been on decaf for years) I personally would avoid it. But if you're used to having caffiene then it would be less likely to affect you in this way. I had to drink 500mls of Lucozade for my GTT and it made me feel dreadful - really sick and heart was racing!
The placenta can't process it either, so can increase babies HR, and if this is already increasing due to contractions, they might think baby is in distress when they're not. (Again this is info from NCT midwife, not from me!)
I don't think the flat Lucozade contains caffiene, only sugar. Which I guess would increase energy levels without having to be concerned about the caffiene effects.
xxxx