Which war killed the highest proportion of British soldiers i.e the highest percentage of the men retained for military service?

Boring , huh?

The English Civil War (1642-1649). The total population of the country was about 5 million. About 2 million men were of fighting age. A quarter of those eligible to fight found themselves in uniform, so there were c. 500,000 engaged: this was the biggest mobilization in our history. Some 85,000 men died in battles and another 100,000 died later of their wounds or disease.

Boring? No, interesting. But I never would have figured it out.