Let's say £100k and only 5% pension contributions.
One person working (£100k): £5.2k take home
Two people working (£50k each): £2.9k take home each, i.e. £5.8k total take home
The couple who split the £100k between them is up £7.2k a year already.
£5.8k. Is that a massive amount of money? Well, let's say 2 parents and 2.4 children. Boy, child and a baby. You want a house with at least three bedrooms (baby can bunk in with the parents for now and the two kids should ideally not share as different genders).
Ignoring the auction, cash-only, sitting tenant and short lease listings, you're looking at a flat in zone 4 for £300k. 10% mortgage, standard 25 years, 4%.
Mortgage - £1,531
Travel x 2 - £414
Council tax - £129
Service charge/ground rent - £65
That's £2.1k down already, without anything to eat, no utilities, no general groceries, and without any childcare.
Let's look for a nursery place for the baby for 5 days a week. £1.2k.
Let's assume we can get breakfast club and afterschool club club. Say £13 a day per child, so c. £584 a month.
Right, so that's £3.9k down just on a roof over their heads, travel to/from work for the grown ups and essential childcare for the kids. That leaves £1.9k for food, groceries, bills (e.g. phone, internet, etc), medical stuff, dental stuff, clothes... for 4.4 people. And this is before even thinking about fun stuff like socialising and holidays. We're just talking the basics of life.
Of course households earning £100k are lucky. But when you start to crunch the numbers, you end up in a position where you have to start budgeting properly, rather than just buying whatever you feel like.
£100k makes you quite happy as a single person. Add in an average sized family and that's when it starts to disappear quickly in London.