Sometimes there is an uptick in the number of children at the very high income end, and some have religious beliefs that lend to large families (or both, as per Mr Rees-Mogg), but why would the two-child cap on child-related benefits (only applied for births after 2017) have even seen the light of day if there was not a negative correlation between income and skill level and number of children?
Because there will always be *some* low-income (which does not equate to 'non-working') families with more than two children, so it was always going to save some money to some extent or other. And low income does not mean 'having children you can't afford'.
Are you one of those people who reads the tabloids and really, actually, believes there are masses and masses of parents out there who just have children as a money-making scheme?
It is also fair to say that the uptick in birth rate has correlated with an increase in migration.
Correlation and causation are not the same thing.
Equally, there are other explanations, for example a strong UK economy both encourages net inward migration for work, whilst simultaneously "native" Brits become more prosperous and more financially able to have children.