I think there’s a big difference between understanding an in-joke like “It’s not Bucket, it’s Bouquet!” and “sod this for a lark.” As you say, “for a lark” isn’t anything strange in the US and “sod” as a British swear word isn’t uncommon in British television or movies.
Faldage gave a data point for “South Leftpondia” and I gave another. I’m in Orange County, California and since the population of my county exceeds the population of the entire state of Mississippi (for example) I felt the difference was worth mentioning.
Just for fun, let’s consider a few numbers. The BBC World News is available to 80% of American households. That translates to something like 220 million people, a number more than 3 times the population of Great Britain. Look at a “small” movie like “Trainspotting” and you find that basically as many Americans saw it in theatrical release as did Britons and when you factor in HBO, which reaches 32 million households or roughly 80 million people then the potential, at least, that “many” Americans have seen it is not out of the question. I’m not implying that BBC presenters ever say “sod” or even that any significant percentage of Americans actually watch the BBC World News. I’m just saying that because the US population is so large and the availability of British television and movies is so widespread, that even a small percentage of Americans watching British television and movies translates to millions of people at least familiar enough with Britspeak to understand something as simple as “sod this for a lark.”
I’m not claiming to “guarantee” anything, I just said that “many” Americans are familiar with Britspeak because of exposure to British television and film. The numbers may be small as a percentage of the total population but they still represent at least millions of people. If that isn’t “many” to you, that’s fine with me. We all see things from different perspectives, eh?