I’m assuming non military casualties were evenly spread between male and female.
3.7 million served in the Army, which is a fairly high proportion when compared to the age range suitable for military service. Add in the Navy and RAF and you get to nearly six million. Those that didn’t serve were generally needed at home - roles like doctors, miners, police, or were too young or too old to fight.
The British, unlike many European countries, had time to mobilise those forces. Had they lost the Battle of Britain and had Germany commenced a land invasion of Britain then it’s likely the numbers would have been a lot lower.
It wasn't until the scam of 'democracy' fooled people into thinking war was against the actual people of the other country that they not only scammed everyone into having such buy-in and stakes for the war but also to view the other countrymen themselves as the enemy. People started viewing the nation of themselves because their laughable miniscule influence of their vote somehow means the government is of them. (Note this was a resurface of course, there were times in history where war was seen as against a peoples rather than of the elite).
Cold comfort. Just decide to not be of Jewish descent then. Who would have known it's so easy to escape the attention of the Gestapo! /s
Of course, at the beginning of every war, some people genuinely believe that joining and defending the nation they live in is in their best interests, but these numbers quickly drop over time. As history and current events show, states start to use forced conscription in every prolonged war at some point.