The protagonist of Radclyffe Hall's "Well of Loneliness" (which I read last year) is a WWI ambulance driver, too -- before I read that book, I didn't know about it, either. Apparently it was the one job that women were considered qualified to do (besides being nurses) ... and they themselves had to fight hard to be allowed to do it; the male top brass, all the way up to the government, were totally opposed.

I'm not at all surprised by that attitude prevailing at the time - UK women couldn't vote until the war was nearly over, Feb 1918.

Same in most of the rest of Europe!

I have a vague notion New Zealand was the first country to give women voting rights.

TWoL was my first encounter of women ambulance drivers, too. Apparently that was based on the experiences of a friend of the author's.
Another thing I didn't know until last year, when I was looking into the Suffragettes, was that the British authorities also refused to let women set up field hospitals. These were to a large extent funded privately and some through the Suffragette movement (different groups), but from what I remember, they only finally were allowed to send assistance when they approached the French authorities for permission.

@Arbie - I think is was New Zealand also.

Re: Voting: Yes, quite possibly New Zealand -- putting all of Europe to shame, not for the first time.

Re: The field hospitals: Yes, Hall actually addresses that in "The Well of Loneliness" (and I think it wasn't just the "ambulance driver" bit that's based on Hall's friend, though perhaps the most notable bit ... and for a person like that, the most proximate choice, short of cross-dressing and signing up as a man).