Think it depends on where you are. Somewhere like Central London you need to dress your best. Same with cocktail bars and wine bars. Anywhere upmarket tends to attract well-dressed men with money so you need to compete. If you go out on a normal Friday night of bars and clubs in a normal British city however it's pointless. If you can get away with it jeans, a print t-shirt and some trainers is best and most comfortable. If you have shoes always wear leather, preferably with a winter or rubber sole cos it's gonna get wet and slippy by the end of the night.