Personally I'd mostly shoot wider than 50mm in a pub. I'd choose either a 28 or a 35. If you don't want to shell out for lthe 35/1.4L, I'd suggest looking either the 28/1.8 or the 35/2.
Take your zoom into a bar and set it at 24 and 28 and 35 and see which of these lengths meets your needs for what you have in mind. Then look at your options at those focal lengths that please you. If you use a flash to supplement the shots, then your 16 - 35 should do fine.
I don't think I could get away with, "Honey, I'm going out to spend $4 K on lenses before we leave for England." Just a bit jealous... :D Congrats - the 35 is on my list... Looking forward to seeing your shots.