If you mean modern day British or American English, there is no answer because it's always changing...people today don't speak the way people spoke a hundred years ago, I would think that if you went to England 500 years ago and heard them speak, you would find that they sound a lot different than any English speaker today...
As for spelling, there was no standard way to spell a word. When dictionaries were wrote, Webster (the for the American dictionary) wanted to simplify the complicated English language, and at the same time, show the British that Americans spoke a completely different dialect than them--thus he wrote his dictionary to show that. He changed the spelling of some words to make them easier to spell, they were spelled more phonetically. So the "-ise" in many words that really sound like "ize" were changed. (realise changed to realize)
Also, British spelling in words like, "favourite, colour, metre" etc. are derived from french spellings (French also use -ise). Webster looked at this and decided that it made no sense to spell "meter" as "metre", because phonetically it is "meet er", not "meet trey". I think Latin also uses an "-ize" ending as well, another reason he changed it.
I don't know how the difference sweater and jumper came about, (is that what you call a sweater? a jumper? lollol weird) I think for those words...maybe they didn't exist way back during colonial times and when they were introduced to different countries, they were given different names. It sorta makes sense, car trunk and boot, sidewalk and pavement, those things didn't exist 300 years ago.
I've heard that the spelling and such changed because Americans wanted to distance themselves from the British following the Revolutionary War, I don't know how true that actually is, personally, I don't think that's the main reason, languages change and evolve over time, especially when you put an ocean between them.