Depends whose toast it is. It could be the toast of Mrs Jones from Abertyleri.
I see what you mean: burning toast in Abertyleri would be news, because the good folk there probably couldn't afford another slice of bread.
Both the regular 'burned' and the irregular 'burnt' are in common use, according to my Longman Student Grammar of Spoken and Written English - but I had to look it up to check!
:-) Thank you - have a lingot. Let's use the irregular then; irregulars have a tendency to disappear. Seems that people are 'lazy' and only want to remember the regulars - is easiest. I consider it a loss though. And after all: one of the main reasons for studying Welsh is because we like to be irregular and keep the old languages alive, is it not? ;-)