It sounds to me like this is about basic skills, or maybe one or two levels above basic skills, but still below actual creation. If it were cooking instead of writing, it might be dicing onions and julienning vegetables, or a step up to caramelizing onions and making a classic bone broth, but getting to the actual creation of food would come later. But you can't get into the class that helps you with the creation of food until you demonstrate the basic fairly-rote skills. And while metaphors are not what I'd prioritize, I can see that for a situation where consistent and fair grading is vital, it would be necessary to have some fairly mechanical evaluation of skill, rather than depending on a bunch of different graders' view of something as subjective as how engaging a story is. Also, concern with the quality of the plot/story itself would mean that the testing is very, very heavily weighted to people who can write at maximum speed, and that's not really all that important. I think it might make sense to remove the narrative creative element altogether, and have practical-nonfiction prompts. But I'm not in charge.
the resources here will help understand exactly what is being asked (these are the 2018 papers and associated materials, mark schemes and so on ) https://www.aqa.org.uk/subjects/english/gcse/english-language-8700/assessment-resources
Do it like we all did it for our foreign language GCSEs Write out some scenarios and stories in advance, complete with point-scoring metaphors, and then memorise them. It's how I passed my French and German GCSEs. I pretty much just memorised what I'd written for my coursework and then regurgitated as much of it as possible in the oral exam. You can do that for English too. Have a story ready in your head that can serve a number of purposes, and prepare in advance how you might tweak it depending on what comes up (funny incident as opposed to dramatic incident, for example - well, ruining someone's wedding cake can go either way depending on how you take it really. Maybe the hen night cake and wedding cake got swapped. Throw in a wailing mother-in-law). Another strategy would be actually just choose a memory. No one will know it's a memory if you don't say so, if they're asking for fiction. That way you're not there trying to make up details from the ether. Have it really well-hashed in your head so you know you'll finish early and can have time to go back over it and put in all the metaphors you like. Another thing to bear in mind is probably that at this level they're not really asking for high quality. They're testing if you know how to use literary devices appropriately. Recently in my year 6 class someone wrote, from the perspective of a WW2 evacuee waiting on the platform to leave London, "We stood there like penguins." (I paraphrase, but the penguins were there.) The kid asked me if it was ok and I smiled and said it's fine. It's a simile. It is more complex than "I am sad". Score! Never mind that the feeling it evokes is utterly inappropriate - it just doesn't matter. The best way to pass these test: download as many past papers as possible. Do them all. I mean all. And then mark them yourself. You go through enough papers, you'll get the hang of how to score points. I must have marked about 100 past SATS paper this past month and by the end I really felt like I knew exactly how to score points on these papers lol. As a real life example, I did exactly this for my own A-Levels. I didn't attend the vast majority of my lessons and never did any of the homework. For English Literature I never even read the text in question (Handmaid's Tale. I did subsequently read it as an adult though). I crammed hard at the end by doing all the past papers and going through each and every available mark scheme. Got As (except for Chemistry. Damn Chemistry.) The Eng Lit exam I got full marks. My classmates did not like me... So... past papers are your friends. Go and do them. You'll be fine.
Alright. Just did my language exam. It took some pushing, but I managed to- sort of- get a story out. The prompts were lovely; write about something you lost or a time you were happy/relieved. So I immediately jumped to writing about the sea. I'm not sure quite what it is about the sea that's so easy to talk about, but it seemed to work. The main character lost her dog, then found him swimming and jumped in to save him etc.: basically, the plot was horrific and all over the place, but you can bet I personified the hell out of the moon.
Had the same thing as you but luckily for me we did half our exam as coursework (like essays) I learnt they give zero mark for creativity and imagination. And we got told ahead of time what the possible questions could be. Like "you've been ship wrecked on an island' or 'your in the woods and something starts chasing you'. We did some practise stuff in the classroom. Then I went away and read some short stories and this helped.