First of all, tap water is not safe but only a few cents for 2L bottle in the supermarket.
Coffee - it's possible to get with milk, European style, mostly in Tunis. It's more common to get sweetened black coffee or the same with milk, but not like a latte. It is, however, very nice. You can expect to pay between 0.7-3 Dinars (from memory), which is less than 1 GBP at very most.