For this you must have a myWaitrose card, free to sign up.
An excellent deal, you must buy anything to get any hot drink free, including mocha. The cheapest item of food is a banana for 25p (same price as buying it in-store). I always choose largest for maximum value. Alternatively, you can upgrade your drink to large, and paying for the upgrade counts as "buying something". It's 30p to do this. Alternatively, if you like syrup, you can pay 50p for a syrup, to make the drink free. You get quality cups and saucers, some cafes have fresh flowers and outdoor seats and newspapers.
This is for people who are lucky enough to be able to get to Waitrose stores with cafes (you have to check individual branches on website and look for "coffee shop" in facilities, NOT "coffee stop" as that is a machine which all have).
Or tea, I think