If you have the offer, what exactly do you stand to gain by resitting those subjects?
I mean the sort of "ultimate scenario" where it's relevant would be if you wanted to apply to graduate entry medicine (which if you're planning on already...is not ideal), in which case there's no benefit to retaking them now as opposed to after you get your first degree (and in fact if you did less than admirably in your resits now, then had to redo them later, may be viewed unfavourably, although as far as I know most GEM courses don't pay much attention to A-levels taken before the degree if the relevant subjects and grades have been achieved after).
Provided you do in fact want to pursue engineering, as you have an offer already taking those subjects is largely irrelevant (particularly as the subjects themselves aren't hugely relevant anyway - outside of the physical chemistry stuff, specifically thermo, there isn't really much). There are probably more productive ways to spend your time - brushing up your GCSE Maths for example, and maybe trying your hand at some of the e.g. AS level mechanics if you're doing well with that, might be more useful to give you a bit of a head start when you begin your foundation year.