You won't be able to get into medical school on results day unless you've applied previously. Medicine is so oversubscribed everywhere that they never have the need to enter Clearing to fill spaces, which is what I presume you meant by getting in on results day. Most admissions tutors for medicine over-offer by a certain amount to account for people who'll decline the offer/firm a different university and have reserve lists for people who've applied for medicine, been interviewed, but didn't quite make the cut. These people can then get in if an unexpectedly large number of offer-holders miss their offers come results day. Bottom line is that you're not going to get into medical school in the UK without an interview.
Virtually every UK medical school requires biology to at least AS Level (with a few exceptions -- see
here) so I would suggest picking that up this year, privately if you have to. You're going to run into real problems if you try to do an A Level in biology after year 13 because most medical schools are fairly hard-nosed about all necessary exams being sat within a two-year study period unless there are significant extenuating circumstances.
There are both gateway courses (for people from disadvantaged backgrounds who're not on track for the standard medicine offer at that university) and foundation/preliminary year courses for people who are on track for the right grades but have taken non-science subjects that don't meet the criteria for normal medicine. OP is unfortunate in that his grades aren't bad enough for the first option and the second option mandates that applicants only have one science A Level at most and, at most places, this one science cannot be chemistry. So those avenues are unfortunately exhausted. :/
To the OP: Your choices at this point are limited to either picking up at least an AS in biology or human biology this year, applying to the few medical schools that don't explicitly require biology beyond GCSE, or doing a degree in something else first and then doing graduate entry, which I don't recommend given your grades. Graduate entry shouldn't even be considered by a school leaver who can get the AAA+ at A Level, such is the imbalance of the pros and cons in favour of the latter.
If you're applying for 2017 entry, you should be able to apply for chemical engineering everywhere except Oxford and Cambridge until 15 January 2017, which is the UCAS deadline for applications to most subjects. However, it's in the same application cycle so, should you apply for medicine, you'd be applying for chemical engineering with a medical personal statement, which isn't impossible, but unlikely to get you offers from some places.
Also, did you mean Imperial College? xD I had to think about that for a while.