To be an actuary in the future

Job description - make certain its really what you want to do - Actuary job profile | Prospects.ac.uk
Then look at the A level and GCSE entry requirements on a range of Uni websites - examples
Actuarial Mathematics BSc (Hons) - 2024/25 entry - Courses - University of Liverpool
Actuarial Mathematics BSc | University of Leeds
LSE - BSc Actuarial Science (lse.ac.uk)
Other subjects that might interest you -
Mathematics, Statistics and Data Science BSc (Hons) with professional placement or study abroad (bath.ac.uk)
Finance and Data Analytics BSc (Hons) - 2024/25 entry - Courses - University of Liverpool
Mathematics with Business Management BSc - University of Birmingham
Then go to University Open Days and listen to the course presentations and talk to current students - again, think carefully about making this choice.

My cousin did chemistry and physics degree at Edinburgh. Now a registered actuary.

Invariably you'll need to do a numerate degree and therefore will need to do A-level Maths. A-level Further Maths would probably be a sensible option as well.
As for which degree you do, any is fine. I've heard of people going in with physics degrees for example. If you know that's what you want to do at the outset then it would probably be sensible to aim for a degree in actuarial science/actuarial mathematics though, ideally one that offers you a number of exemptions from the relevant professional exams (e.g. at LSE or Southampton).
Note that the universities which have preferred subject lists (UCL and LSE primarily) both consider accounting a non-preferred subject, so I would probably not suggest taking that to A-level.

Thax for the reply ❤️. I'm planning on doing eco before actuarial science (eco is my fav subject and since I can study to be an actuary with any kind of degree, I want to study eco more). Aside from maths, what subjects would help me more?

Economics as a degree? I don't think "any degree" is acceptable for all recruiting companies - I think they normally look for numerate degrees (e.g. maths, CS, engineering, physics), and sometimes economics is included in that range and sometimes it isn't.
Just looking through some listings now, APR accept any degree but make reference to the mathematical necessities of the role, EY require a numerate degree which they include economics as one such example, WTW sometimes include economics for actuarial roles as a suitable degree but sometimes do not, and Metlife specifically require a degree in actuarial science, mathematics, or mathematics based sciences. If aiming for economics as a degree you therefore may want to consider strongly joint honours courses in economics and maths/stats or something like the MORSE course at Warwick for example.
In any event for any of those degrees as noted maths is the primary requirement, and FM would be highly desirable for some of the more competitive courses (possibly required for a few e.g. LSE you're expected to do FM if your school offers it). Beyond that for economics or maths (or CS) any third subject is fine. If you were so inclined to the physics or engineering route A-level Physics would obviously be necessary, although if that's not of interest you can safely skip it.

Economics as a degree? I don't think "any degree" is acceptable for all recruiting companies - I think they normally look for numerate degrees (e.g. maths, CS, engineering, physics), and sometimes economics is included in that range and sometimes it isn't.
Just looking through some listings now, APR accept any degree but make reference to the mathematical necessities of the role, EY require a numerate degree which they include economics as one such example, WTW sometimes include economics for actuarial roles as a suitable degree but sometimes do not, and Metlife specifically require a degree in actuarial science, mathematics, or mathematics based sciences. If aiming for economics as a degree you therefore may want to consider strongly joint honours courses in economics and maths/stats or something like the MORSE course at Warwick for example.
In any event for any of those degrees as noted maths is the primary requirement, and FM would be highly desirable for some of the more competitive courses (possibly required for a few e.g. LSE you're expected to do FM if your school offers it). Beyond that for economics or maths (or CS) any third subject is fine. If you were so inclined to the physics or engineering route A-level Physics would obviously be necessary, although if that's not of interest you can safely skip it.

I don't plan on taking science subjects so ig economics, accounting and further maths will be more suitable. What do you think would be another subject that will be fitting "if" I take an additional subject? Just curious.

You can't take further maths without maths. The former requires the latter. There isn't any benefit in doing four A-levels either so if aiming to do maths/FM/economics I'd just not bother with the accounting A-level - it adds no value here.
Otherwise literally any traditionally academic subject is fine - you probably gain less from vocational or applied subjects (such as accounting) as some unis consider them non-preferred. You could do classics, you could do psychology, you could do a language; geography, history, English, whatever. Universities don't actually care about subjects beyond required ones outside of the few who do prefer traditionally academic subjects. Whether you do accounting or not is immaterial (except as noted, it's not going to add any value to a combination of 4 subjects and may make you a weaker applicant for some unis if taken as part of a combination of 3 subjects.
University admissions is considerably less complicated than you're making it out to be. Do maths, do FM, then pick another academic subject (if that is economics then go for it), and you're set.

Oh! I was keeping accounting on purpose as it is also very numerical... I'm honestly not that comfortable with that subject. Thank you so much for assuring me, you're a lifesaver

Don't think I'd really consider it that numerical as a subject as far as A-level is concerned. Honestly outside of maths and FM I don't think any A-level is enormously mathematical. Even the ones that do involve a lot of maths work (e.g. physics) are just using GCSE level maths (A-level Physics is all algebra based rather than involving any calculus...). Plus unis don't actually care about whether your other subjects are "mathematical" or not provided you meet the actual subject requirements of e.g. A-level Maths and/or FM as above

I have physics and chemistry in my o levels as well as accounting and economics. I'm dead fixed on both economics and fm because I'm very strong at these. As for phy, chem or acc. I'm stuck there... Three subjects are my fav and I'm confused about what I should pick from these. This is the main reason why I made this post.

Just do maths, further maths, and economics then. You have your three subjects and they're the ones you like. You don't need to take another subject, at least as far as UK universities are concerned.
If your school requires you to take a fourth subject in year 12/to AS level, just pick whatever you don't mind and try and drop it as soon as possible.
Out of physics, chemistry, or accounting none of them will be better or worse than the others as a fourth subject. If you don't take further maths and just do maths and economics plus one of those three, then I'd suggest physics or chemistry since as above, some UK universities consider accounting A-level as "non-preferred".
As a general note though A-level Physics does give you some other options for degrees in case you change your mind and decide you'd actually rather do e.g. physics or one of the many fields of engineering. It's not specifically better for anything not requiring it though.