Sparkling mineral water with natural flavors (la croix, perrier lime or lemon, san pelligrino). La Croix comes in many flavors, all with 0 calories (and no sugar, fat, or sugar fillers). It's just like drinking water, but with flavor.
Go to Tesco's drinks section there's a variety there, my fav is Cloudy Lemonade at 38p for a 2 ltr bottle you can't go wrong.
There's Crushed Orange, Limeade, Cherryade, Appleade. Take your pick...Cream Soda, Cola. Have a Look.
i am with slimming world and am always looking for low cal drinks bovril is a free drink but only if it is in the jar cubes are not cal free also if you get a lemon put a few pieces in hot water with a bit of low cal sugar that is quite nice
No Added Sugar squash has virtually no calories, Robinson's Fruit and Barley tastes the best I think. Any fresh fruit juices are low in calories and a small glass will count to your recommended five a day fruit and veg intake.
Some of the Innocent Smoothy flavours can be quite low in calories just need to check the label.