>76,869 British nationals were recorded in the prison population at the end of the first quarter of 2024. The remainder of the population was made up of 10,422 foreign nationals, making up 11.9% of the total population, with an additional 578 having no nationality recorded.
It seems the 37% figure may have come from an entry in the table breaking down those non-British nationalities by country:
>All other nationalities 3,899 37.4%
https://en.wikipedia.org/wiki/United_Kingdom_prison_populati...
At one extreme, if the population of London happened to be almost entirely foreign born then it would seem that UK citizens are working overtime at London crime.