What percentage of UK cattle are kept indoors their whole life

This is called "zero-grazing" or "continuous housing"
Bear in mind that % of herds is not the same as percentage of cows, because larger herds ar emore likely to house indoors.

9,631,892-1,056,000
(1,056,000*0.1)+(8,575,892*0.04)
448,635.68/9,631,892
802+141