[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.