When I was tested for Gestational (over 20 years ago) a plain glucose drink was provided. It wasn't very nice to drink, but should still be available. The drink has to have a measured amount of glucose in it and that's why they use lucozade.
I would insist that you are provided with a suitable drink. It wouldn't be good for you or the baby to have an allergic reaction to the caffeine. Glucose powder is available to buy from a chemist and this could be dissolved in water as long as you knew the correct amount.