I am not quite at 300k but not far off.
Firstly masses goes on tax and NI. You do not have any personal allowance at this level, and you have virtually no pensions allowance so you are paying tax on your pension contributions too.
If you have young kids, you spend a lot on childcare just like anyone else (except you don’t get tax free childcare). If you are earning 300k you are probably working all hours God sends, so you will be paying for some kind of wrap around care. Probably a live in nanny, because who else is going to cover for you if you have to pull an all-nighter or go to Frankfurt on a business trip at short notice. And if you have a live in nanny and kids you need somewhere big enough to put them in. And if your job is in London, which many 300k jobs are, then that home is going to cost you a fortune in rent or mortgage payments.
Older children - probably if you are on 300k, unless you are ideaologically VERY motivated, that means private school fees, so anywhere between 20k and 40k per child depending on age and school, from after tax income. Many of my friends have three kids, so that’s maybe 100k a year, more than half your take home pay, on school fees alone.
Finally there is the big expense people don’t often talk about which is alimony/maintenance. It’s not uncommon for people who work long hours and earn a lot to be on second or even third marriages, and (while things are changing now) there used to be a presumption that as well as child support, ex-wives should be kept in the style to which they’ve become accustomed without needing to get a job. That can get very expensive, especially if you’ve started a second family with a new partner.
Once you’ve done tax, pension contributions, a bit to savings, childcare/school fees, mortgage/rent and maybe child maintenance and alimony, there’s no room for “staff and assistants”, or some of the other very expensive ideas mentioned on this thread.
In my experience people earning at this level tend to be time poor, so you often see them spending their spare cash on nice holidays to exotic places - a kind of compensation for not seeing their families very much.