I applied for physics, so I'm not very sure about engineering in general.
Some advice I had given to start doing some general supercurriculars now in Y12, e.g. some irl and online Olympiads (SMC, ACOT, Physics Brawl), read a couple chapters from books that can also serve as chem/physics revision and deeper reading, or a couple chapters from a chemical engineering intro book. [I recommended doing a lot early in Y12 because that's what I did and it really helped cuz I could focus on A Levels in Y13 and gotten early offers, then again I applied Oxbridge so it's pretty much necessary.]
Not sure what else I can say tbh. Any ideas?