I've changed my opinion on one thing, thanks to this thread which has sent me down the rabbit hole of watching historical phonology videos, I've realised we have a fairly good idea of what people's accents long before the era of sound recording.
I originally wanted to find out whether it was sodder first and the Brits started pronouncing the L or the other way round. Not only have I not found the answer, there doesn't appear to be strong evidence either way, but its raised more questions.
Why have certain sound changes only propagated over a certain geographical distance, then run out of steam? Take the strut-foot split for example. In early modern English, the U vowel in the words foot and strut sounded the same, so the two words rhymed, yet in the mid-17 century, Us split into two different sounds. Interestingly this did not affect Northern England. There's an isogloss running through the Midlands. which divides where the words wood and blood rhyme and don't rhyme. I'm just south of this isogloss, put doesn't rhyme with strut for me, yet I recently asked someone from the next town to the north and she said they do. Contrast this with non-rhoticity, the lack of the R sounds, which started a little later yet spread further north, but not so much in the south and west.