I mean, the slip itself is inherently funny. I know it's a deadly serious matter but sausages is just a funny word and for many Brits, the seriousness of the matter makes it even funnier that he said it. Nothing to do with their phallic appearance. We love to see authority figures making arses of themselves. And we've always found sausages funny.
But there is more. We have a media which is disproportionately weighted towards right of centre politics. They've spent the last 5 years trying to convince everyone that the right-wing governments we've had in that time were serious politicians with serious ideas. Much of the media took a kicking in our last election as well as the Conservatives did. They're used to their readers and viewers doing what they are told to. This time they didn't.
So the majority of the media is in a mighty hurry to prove that voting Labour was a big mistake. The sooner they can get that story to stick the quicker the return to 'normal' i.e. right-wing politics.
There's also the fact that Starmer positioned himself as the sensible, competent alternative to Tory facetiousness. This slip up makes him look daft.
That's why they're making a much as possible of every Labour mis-step.
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The Andaman Sea, the Andaman Sea, Oogily boogily, the Andaman Sea.