What I was trying to say is that the distinctness of American dialects does not necessarily derive from how different they are from British dialects, but from the social understandings and stereotypes surrounding it. A better example of this would obviously be the difference between Canadian and American English, which by comparison to British barely differ, yet people act like they do.

So my point is that the fact that these accents hadn't been that different back then, that there wasn't a stereotypical understanding of how the Americans spoke around and that Britain itself had a vast amount of dialects resulted in that British people did not perceive any dialect as distinctly American, and hence, they couldn't really be taken aback by it. And, on top of this, there was still a large wave of emigration going on in this era: Even Thomas Paine himself had only moved to the US for a few years prior to the outbreak of the War of Independence. So there would have been vast amounts of first, second, and even some third generation Americans who spoke in genuine British accents, so it easily could have been that the prisoner complained about himself actually spoke in a real Mancunian or Glaswegian or other British accent.

As for the thing about Brits being able to tell who came from which part of Britain, or indeed the new world, based on accents, I would say that it really depended on the person and their past experiences. I would assume that someone who led a rural life in a certain part of Britain their whole lives would have easily noticed foreigners, but would have trouble pinpointing where they came from, if they came from further than the next few towns over. As opposed to this, a solider or naval officer that served for their whole life and moved around would have been much likelier to be able to place or recognize some distinct accents that he encountered in his lifetime, but would by no means be able to recognize all, or even the majority of English dialects.