black tea, black coffee, green tea, water, spring water, herbal (not fruity) teas, no squash, and definitely no orange juice! (it will have a fructose content).
Some of the other VLCD suppliers do compatible flavourings for water - I'm looking into investing in some.
IF you're desperate, the following are just about compatible with ketosis, anecdotally:- Coke Zero (although not diet coke), Perfectly Clear (only the strawberry and kiwi flavour) and Dr Pepper Zero, allegedly. Basically, diet fizzy pop without citric acid in. (Coke zero has phosphoric acid).
Although fizzy pop on a practically permanently empty stomach is not entirely recommended...