Chemistry is a bit more of the tough side, but it should be doable as long as you "crunch through the past year papers and memorise everything from the book". Yeah, I am a complete idiot in chem, but I still manage to get an A just by "working like an asian".
Retake? What retake? You are only allowed to retake exams if it means that failing those will result in your expulsion. No, unlike A-Levels, universities cannot take these "retake" nonsense, the find them insulting (I failed? oh, retake :P). A bit like life, sadly. January tests only account to 5% of the marks of this year anyway, so its no biggie. If I fail more than I should, I will have to explain myself to the senior tutor etcetcetc. Well, good luck on your Physics and Chem, maths should be a piece of cake for you, (so should physics and chem, but those papers can be real b!tch3s!).
Hi Bo££! I have just successfully screwed my January round of tests, and no doubt you are still powering through your set of January tests, so when you are done with them, I will take questions you have about Imperial College (and university life)!