Hey,
Best thing in my opinion to do is wait for the ACA trainee scheme and start your exams with your new employer. Big 4 and the other large practices typically do not hire trainees who have already self-funded some of the exams. They expect candidates to have done zero exams and will fund your training. If you were really really keen to get a head start, then you can consider getting a hold of a study manual and question bank for some of the certificate level exams, however this is by no means necessary as they will teach you what you need to know at college. Also, reading the study manual can be quite pointless and boring. If there is one exam you might want to do it for, then I would say get the Certificate Level Law resources. The module is pretty much a learn and churn and the study manual isn't that long to read. Material is really dry though. Also for the law module, you don't really go to college, it's normally self-study. If you study with a tuition provider, they'll give you some additional e-learning resources for law but you might even be better off just reading the study manual and going through the question bank.
Another thing to check is for exemptions. You may have exemptions for some of the earlier exams with your degree because you did management with accounting, so it is worth checking this. When you do get your ACA contract, some firms may allow you to take those exemptions however others may not. To check for exemptions, you need to go onto the ICAEW website and check in the CPL section.
So in summary to answer your questions :
1- Should I start studying from January for the ACA certificate exams whether it is alone or with a tuition provider then when I join the training program I will have 6 exams less. - No need start studying for the exams. The courses are fairly expensive and it may work against you when applying for certain trainee schemes, as larger firms in particular expect candidates to have done zero ACA exams.
2- If its worth studying for the ACA with a tuition provider, what do you think about BPP and Kaplan? which one should I go for, they are both between £3,000 - £4,000, or should I go for it independently as its certificate level exams. - Again, no need to self fund. Find a ACA contract with a firm first and let them cover the cost.
3- Should I just wait for the training programs as they will cover all the costs? as I think big firms want fresh students. Yes, most of the larger firms expect trainees to start their ACA contracts fresh with them.
4- Should I buy the ICAEW Books for the certificate level and go through them in my free time and just wait until I apply for the training programs in 2021? - This may help, because when you do start the courses, depending on the company you work for you are expected to understand the material thick and fast. There can be a 7 day window between learning the material and completing the exam for some modules. Law is the certificate level module which is typically self-study (unless something has changes over the last 3 years), so you may benefit most getting a study manual and question bank for this exam. Similar thing could be said for Business, Finance and Technology. Not necessary though, it could prove to be a waste of money as your company will pay for fresh resources for you. Also, when I mean buying resources for Law/ Business Finance and Technology I mean just buying the study manual and question back, which I think costs £35 for each certificate level module. No need to pay for courses at BPP/Kaplan.
Other things you could do in your spare time, is to just brush up on double entry accounting. This will help with the certificate level accounting module.
Hope this helps.