I heard that this phrase actually found its origins after
World War I -- enough
English soldiers were lost during the war that
England experienced a bit of a population problem, for a while, so the English ladies had to work overtime.
Exhortations alternate to the one given above include "
spread your legs and think of England" and "lie on your back and think of the
Queen".
AFAIK, no woman has ever experienced orgasm while contemplating any of the Queens of England. But I could be wrong.