[mathieuh]

I am not a linguist, but I do speak BrE natively and to me "-t" and "-ed" are used in different context. The "-t" ending is used for adjectives ("dinner is burnt", "that child is spoilt", "it's spelt like this"), and "-ed" is used for conjugations of the verb ("I burned dinner", "you have spoiled your child", "I spelled it like this").

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.

[stordoff]

Also a British English native speaker, and I'd generally use -t for both cases, though sometimes I'll use -ed. I seem to slightly bias away from "spelt" though, possibly due to it having another meaning. I suspect it's regional, and -ed feels slightly more passive to me (similar to 'it burned' vs 'I burnt it').

[ferzul]

I have heard it described that -ed is used more often when the speaker is conceptualising the time it took, but -t is used as a default.

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.