Usually it's 20% for good service anywhere in the world. In London taxi driving is actually a very skilled occupation where licenses are only given to those who meet rigorous standards. Your driver will probably know exactly how to get anywhere in the Metropolis.
If the taxi driver was in the middle of the road then it wold be the taxi driver fault but if the taxi was on the side of the road then the dumb taxi driver should know that the rule is to open on the right side either way! duhhhhhhhhhhhhh