While I can't speak for any other country but my own, I have the following to say about this issue concerning you Brits.
First my own personal view and take on it. I happen to know quite a few Brits who are younger than myself by about 10 years or so plus, and I have known them to be not only very smart, but brilliantly funny. I haven't noticed them to be overly prudish, at least not all that much more, if any, than any other country is. From what I gather the English might not be quite as open about naughty things as we Americans are in general, but times are changing as they have changed for us over the past 50 years.
The Brits have their Kings and Queens and the entire royal family respectively who are taught to be, well, for lack of better terms here, prudish. It was not until the worlds beloved Diana generally speaking who all on her own started to change England's outlook on the Royal family and their traditional ways. Along that time there were a few other members of the Royal family that also started breaking the Royal traditions.
As someone who loves to study culture and traditions of the world, this time in the lives of you Brits drastically changed, for the better in my opinion, and broke many of the stiff and or prudish traditions of the Brits culture and way of life as they knew it. The English are in love with the Royal family, there is absolutely no question about that which is fantastic. They Royals up until Diana were very prudish and many, probably most of the older generations still are. They were raised to be proper, polite, correct, reserved and a slue of other things along those lines. Which as role models for their country and basically the world as well, they needed to be and should have been. It makes perfect sense and is psychologically in line as to why the Brits for generations have as a whole been a bit on the prudish side. They were emanating the people they looked up to the most.
So with Diana came change not only with the Royals but with the country in general. I happen to have great respect, and admiration for her as a person, as a world ambassador, and as a woman. Personal feelings aside she pretty much single handedly changed a nation and their ways of thinking. She stood up for herself, her boys, and for women in her country. She bucked the Royal system and did what she felt was the best for her family. Because from the time Diana stepped into the spot light as a soon to be Royal, girls of all ages started to follow her, emanate her. They wanted to be her, be like her, look like her and whatever else like her that they could.
While everyone in the world was falling in love with Diana, the little girls in England which would be basically my generation and younger, were worshiping her. They finally had a role model that wasn't your 60 year old Queen, and I don't mean any disrespect by that. However you Brits can't deny that your current Queen is really the epitome of prude, which again is fine. Her generation was raised that way and children learn what they live. If you are only taught to act one way and you never are exposed to any other way then that is all you know. The girls in your country up until my generation only basically saw one way of acting, one way of thinking. Their moms acted that way, their grandmas and every generation before them. It is all they knew.
Diana taught the girls of England to be strong, stand up for themselves, for what they want, deserve, and for what makes them happy. She pretty much single handedly changed a nation. Not many people can say that you know. So moving to the United States, who is it we have seen most from your country? Is it the normal ladies that you can find here on EC? No, it's your Royals, and until Diana we were only seeing your older Royals who, for lace of better terms are prudish.
Also, something else many people are forgetting. In America again we have limited exposure to real life over in England. What we were exposed to besides the 6 o'clock news about your Royals was basically Monty Python and Benny Hill. Both of those shows portrayed most of the women in your country as extremely prudish. So not making excuses for us Americans, but if we only see one thing and are never seeing anything else, that is all that we would think as the truth. See what I am saying?
So please forgive us Americans and our narrow mindedness on this topic. Hopefully you Brits can help educate us and other countries and help to break some of the stereotypes associated with the women of your country.