If they come and live in 10M house it's 300k/year right there if you tax properties/land (you can use progressive rate so it's higher on most valuable properties). Add consumption taxes and every rich Chinese/Arab kid is sponsoring welfare for few dozens people per year.

The yearly council tax of a £10M house in central London is the same as a £320K house:

https://www.westminster.gov.uk/council-tax/council-tax-bands...

So where did you get £300K/year from? Or are you saying that is what it should be?