@Yazhi and @lettie9 - sorry, made lots of mistakes in reading and posting late last night and we're at cross purposes (my mistake).
I'm not saying a person definitely can't earn 6 figures by age 30 unless they're a banker. I earn six figures as a digital contractor. I came to it late but there are others in my industry on the same money much younger, though most of the young ones are more like mid-30s as they'd still need some work experience before being able to go contracting. Obviously it's possible to get that level of salary before age 30 but I think if we were to look at salary surveys etc those people would be a very tiny percentage of people overall, even in London there can't be that many jobs at those figures percentage wise.
In my sleepy haze last night I got mixed up between 6 and 7 figures and salary/house. I was focussing only on the house, and the house is 7 figures. I guess I'm talking in broad brush strokes but the whole of London has those 7 figure house prices and I don't think the whole of London can be made up of people who reached 6 figure salaries, and also had their other half reach a 6 figure salary both by a young enough age to get a 25-year mortgage of £780k? Yahzi you can afford yours by selling that stock, so it's not on your salaries alone right?
So as those 6 figure types of people will be a small % of Londoners, the rest of the people buying those 7 figure houses must have had inheritances, bought and sold previously, sold stock, etc etc.
I lived in London for many years and most of the people I knew there at the time were in TV, or social workers, or nurses, and none of them were on 6 figure salaries. I moved out 17 years ago and can only assume that the makeup of London must have drastically changed as now the houses are 7 figures not 6 figures, and not that many people percentage wise earn 6 figures and also have their partner earn 6 figures. So the house buyers must be getting the ££ from somewhere else.
Hope that makes more sense! Note to self - don't read and post on MN when half asleep!
ps also here for tips on which jobs do get you to 6 figures nowadays by age 30 so can encourage kids in that direction!