So if you're changing subject then in principle most universities will not necessarily hold a "false start" elsewhere in a different subject against you. If the subject is very closely related though that might raise some questions. I think in this case biochemistry is fairly distinct (as in the UK biochemistry degrees are fundamentally bioscience degrees).That said your rationale for choosing it in relation to the area you struggled with in your chemistry degree doesn't seem to quite tally for me - you will need to do maths in a biochemistry degree, and in fact in any science degree.
What specifically is it from the chemistry degree maths that you struggled with? Is this really something you would not need to manage in a biochemistry degree? What other courses have you considered? Is your rationale for choosing biochemistry just "it's chemistry with less maths" and if so, is this actually an accurate reflection of the course (it's not)?
Note I would highlight - having the CertHE or not is not the issue here and I don't believe the qualification itself won't impact on whether you are accepted or not. It's the prior study which is a relevant consideration (and you would need to declare that anyway - whether you got the exit award or not). Whether you withdrew from the course after 1 year without receiving an exit award or opted to receive the CertHE I don't believe would make a difference here for admissions purposes (although, it might for funding/SFE...more below on that). So I don't think it will negatively impact an application per se.
However, equally I don't think the CertHE is going to change the entry criteria in terms of A-levels in this case. So you need to be looking at courses your A-levels correlate to the entry criteria for; if you want to apply to courses with higher entry criteria then as above you'll need to contact them and see if your CertHE may hold any water, but given in this case it's an exit award I don't think it will make the difference you're hoping there. That is, I don't think it will positively impact an application which seems to be what you are hoping for.
In terms of SFE funding - you'll have reduced funding due to prior study, which assuming you just did the one year will eat up your "gift year". Now I believe that the CertHE should still allow you to get the full funding (minus the gift year due to the prior study) for a BA/MSci/etc as it's a lower level qualification, but I would strongly recommend you confirm this with SFE just so you fully understand what you will be expecting for next year funding wise.