There are 26 bars in London and you have to drink at least once at every location with THE SAME character which is not mentioned in the description. Simply visit bars borough by borough and try to remember the name of the bar you've been to. Remember to play the dart game, too if you see one in the bar for another achievement. Luckily, every dart game in London is located in a bar. I recommend you do these two in a single walkthrough because both are easy to miss. For dart games, simply play solo mode and complete it would count.