£550K is the average, not the mean [1], not sure where you got that number. Median salary in London is around £40K [2], and the average around £60K (don't remember where I read that figure, it might be wrong). Unless you are buying alone, with two average salaries you can buy an averagely priced home. You don't need a £100K salary (percentile 97!) to be able to afford a home in London, stop with the pointless exaggerations.
Note 11 of [1] states that the geometric mean is used. So this would be different (and lower) than the 'average' in the usual sense of the arithmetic mean.