I'll drink anything with caffeine in it... it's the only vice I have left!
Strong tea or coffee to start the day, then at least one or two bottles of diet soda to make it through, and a nice normal cup of tea when I get home. The only concession I make is to use sweetener instead of sugar, as long as it's not saccharin or stevia it's not that bad.
Most of the flavoured instant coffees and coffee beans from Whittard are sugar free and with a bit of cream and sweetener make a passable substitute for Starbucks when I want to spoil myself. They also have flavoured green and black teas that are a nice treat, best to ckeck if they have dried fruit in them though because it will add a few carbs. Their peach flavoured tea is wicked good made as iced tea in the summer.