There is nowhere on the Cambridge application form for you to write your A level or GCSE results, so they won't be able to take these into account, unfortunately. I'm sure Oxford was the same, but I can't remember! This is a link to a (rather long) thread I started back in 2017 which might have some helpful tips when it comes to applying and the application process. It's a few years old but I'm sure you'll find some good tips from people:
https://www.thestudentroom.co.uk/showthread.php?t=4207500 Don't worry too much about first-year grades. If you pull up from now you'll be in with a shot, and it shows that you are able to recognise your weaknesses and build on them, which is always a good thing. Don't forget: Covid has ruined a lot of people's academic performance this past year or so, and is a totally valid reason for not scoring as highly as you'd have liked. Your referees can also mention the impact of Covid in relation to your first year results, but only if they can see a stark improvement in your second and third year results. You want them to be able to say "yeah sure they fell short in first year but this was only because of Covid", but if your grades remain the same throughout second and third years it'll look like Covid wasn't the reason you fell short in first year. Now is the time to prove your true potential, and you have more than enough time to improve. Covid isn't going anywhere any time soon, so you need to find a way to work around it now that we are more au fait with distance learning/working than we were this time last year.
GradesBasically, the way they analyse your grades is by looking at your transcript. So, when you apply, you have to submit a transcript of your grades to-date. The university will produce for you a list of every module you've sat, and what the overall raw score is for each one, such as 65.7 or 53.1 or whatever. This makes it easier for Oxford, Cambridge or whatever other universities you apply for to establish your true potential, because a score of 61 and a score of 69.4 are both 2:1s, but the 69.4 is a much higher raw score than the 61. That is why they don't use 1:1, 2:1, 2:2, etc. but the raw scores, because they give a much more accurate reflection of a person's capability and potential.
Also, they can see how you perform across the board, so if you are strong in other modules and get high 2:1s and firsts, but get 2:2s in a couple of modules that bring down your overall grade, they can see that you are a strong candidate but that you struggled with a particular area/certain modules. If you can show them that you improved in second year, by getting high 2:1s and hopefully some firsts in there too, that will help massively. Put all your efforts into getting a first, and if you fall short of that then you should still be in the high 2:1s. If you aim for a 2:1 and fall short, you are in the 2:2 bracket, so always shoot high. There's no such thing as "naturally able" or "naturally clever", just people who exhaust all options to help themselves score big. So, become the kind of person who sniffs out every opportunity, every bit of support, and work your little socks off.
Building relationships with academic supervisors, guidance counsellors, etc.Make sure you are present in every lesson you have. I cannot stress how important this is. If you have tutorials/seminars then make sure you participate by offering answers and engaging in discussions, get feedback from your academic mentors or whatever equivalent you have at Manchester, and form relationships with your teachers and supervisors. You will need references when you apply for your masters, and you want them to be glowing. You want them to be able to say that you are on track for a high 2:1 or a first overall, and the only way your referees can say that is if they can see your grades improve in second year and see that you are putting in the work. They are more likely to polish your reference if they like you and appreciate you and are able to put a face to the name.
Forget first year. It's gone, done and dusted. Focus now on second and third years. Create an action plan. Create a list of all the people/support networks at university that will help you improve your grades. Meet with your mentor/supervisor/whomever and explain that you are worried about your first year results and really want to have a shot at Oxbridge, and ask for any advice they have. That is their job, and they are bloody good at it.
Sorry for such long answers, but I really want to flesh things out a bit so that I can help you. I hope what I've said makes sense, but let me know if you need any clarifications!