Figures I’ve seen say over 700,000 casualties in the British Army alone.
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.
I’m assuming non military casualties were evenly spread between male and female.