The trick is there *isn’t* a ‘base’ English, Huli. Theres NEVER been a single ancestral English from which all others branched. It’s a bit like asking ‘what automobile is the ‘base’ car?’, there just isn’t one single one. Even if you go back a millenia or two before there really *was* ‘English’, you’re still having folks learnignt he language differntly from one another, and passing that on as regional differences, etc. Linguistics is a VERY fluid concept.
Pat of it is due to travel. Back in Colonial days, you’d have a regional breakdown of accents similar to what you see in the UK. The journey westward, the railroad, and later automobiles all contributed to homogenizing the language. The current accent set is due to folks not having much reason to travel outside their large regions, rather than capacity.
California got an extra helping of this blending due to the gold rush, with folks coming from all over to try and get rich.
The one thing I’ve noticed is that a lot of the ‘words that are different in UK English and US English’ are the result of.. well.. having a big ol’ ocean between the two. Words that were made BEFORE Colonial days stayed the same, but as time went on, new inventions got their own regional nicknames. Even when travel between the two by sea became commonplace, you really didn’t see the ‘naming of things’ sync up until after WWII, when airplane, radio, and later television made it all possible to share ideas faster. This is why we have different words for car parts, but not for computer parts.