Well... if you can somehow sneak into the UKMT headquarters and have a peek at the question paper itself...
(obviously joking, but you asked for a cheat)
Now just my two cents, for real.
(1) The "cheats" in maths are often developed upon repeated practice and (partially) active understanding.
So, uh... do the past papers - and redo them, and redo them again, I guess?
The thing is the solution often starts with the words "notice that". Well you might ask "how on earth does someone know to notice that?" Unfortunately, no guarantees - but by doing lots of maths you can increase your chances of being able to spot the trick.
(2) What do you think is the hardest part about SMC? It might not be the maths, but something else...
For instance, maybe you can do the questions in 24 hours without looking at any hints, but not in 90 minutes?
(3) It's absolutely fine if you think the questions are hard. They are meant to be hard (or really, hard if you can't "spot the trick" IMO). Don't give up at this stage (but also give up accordingly during the actual test, refer to (2)).
No A-Level maths/FM knowledge required. And to be honest, calculus is not going to help here.