You can have coke zero as it contains no citric acid which affects ketosis. Diet coke does contain citric acid. But it should be drunk in addition to your water intake, your kidneys still need to flush through the toxic ketones that ketosis produces.
There is a sticky somewhere of drinks that are ketosis friendly. Like sainsburys do a strawberry and kiwi fizzy drink which is safe. Tesco do a strawberry flavoured still water which is safe.
If the ingrediants say citric acud its a no go, malic acid is safe.
Though its not cdc approved, many other vlcd's say its fine. Cambridge has to be firm and have clear guidelines so isnt likely to advocate cheats
I cant bear water, so I drink sugar free squash, which isnt allowed and may be slowing down losses but its the only way im going to get 3 litres a day down me