Sometimes it is easier to go with the flow and just accept these exceptions. I think broadcasted is right too, but I hear broadcast more. But it really doesn't matter which one you chose. I don't think it is an American/British thing - and who would care, anyway (if an American doesn't know where the bonnet is he needs to watch more "Downton Abbey"!). Cast and cost don't add "ed" to their past participles, so why not put broadcast with them, and then you can remember them all.
One thing, Victoria. I would have to look it up to see if it was OK to say "May it be....?" a century ago, but people don't say that now. They say, "Could it be...etc." Actually, someone on "Downtown Abbey" might say, "Might it be...?" Yes, that would be OK, but outdated. I wouldn't use "May it be...?"