See, to me 'spelt' just feels wrong, but 'ise' does too. I've found that I generally lean toward US English, but then they're the weird interactions with my own language (Dutch) that make me uncomfortable with words like 'aluminum'.
I honestly don't care either way, it's a bit jarring to me to see American spellings of things, and I've done work for gov.uk before where the style guide says to use British spellings, but I think it's just being obtuse if you have some evangelical adherence to either way.
That would mean “it burned” brings to mind images more like “it was burning” and “I burnt it” brings to mind images of having something that is burnt.
Neither of your examples are passive btw.