Can I go into Law with comparative literature?

If you're talking about a law conversion or going into law as a career after an initial degree in comparative literature or modern languages, you should be fine. A multitude of people with different degrees go into law afterwards and it won't really be a problem unless your degree is something truly niche, rubbish or something like that.
Don't worry about it, honestly.
As for competition, it's hard to say. Competition would be nasty with a law degree, just as with any other degree. I don't think what you want to do would decrease your chances at all, but that isn't to say that there won't be quite a bit of competition anyway.
Either way, your chances are almost always better by doing something you want to do and will actually enjoy.

Do you think there's any point in doing law if I got BBB at A-Level? I'll basically be filtered out automatically, even if I got a degree from a RG or got a first

Okay, so there are a few things to unpack there. So, when you say doing law with BBB at a-level, are you referring to it as a degree or as a career choice?
BBB at a-level (without contextual considerations) I don't believe would get you into a RG uni for law. Although going to an RG uni for law isn't necessary to go into law as a career.
Secondly, BBB at a-level may hamper you beyond that. Kind of. So, if your plan is commercial law, many firms (at least at the just-graduated training contract stage) have a-level requirements, which typically sit around AAB, I believe, although not everywhere had this, most places do. I would imagine, although I don't know, that if you have a really good degree this can be overlooked, and once you have a masters they may have moved on from a-levels - certainly this would probably be the case if you have years of job experience in the field before that. Either way, having BBB at a-level is likely to, at least soon after graduation, effect your competitive stance. You'd need to work harder to make yourself stand out and even then I don't know how willing some firms are to be swayed.
Beyond that, I think that if you want to study Law at degree level, you should do so. No matter your a-levels, there's not much point in adding a few years to your career path if you want to study Law and want to go into law. If that's what you want to do, I'd go for that.
Also, having studied at an RG doesn't automatically make you a shoe-in, although a first certainly may, even if you got B's in your a-levels.
Finally, a-levels may get you filtered out as a recent graduate, but both beyond that, and if you are an outstanding candidate, these things can and are overlooked.
Have you sat your a-levels already or are you making a guess at results?

I have achieved BBC in 2023, I'm taking a gap year to figure out what I wanna do, I took up a bunch of courses, workshops and a-levels (which I dropped) and got an extra A-Level (Psychology) which I may get a A/B at which can get me into Comparative Lit/Modern Languages in UCL/KCL.
But now I am realising what is the point of doing a degree that can get me into a average job e.g. marketing that any other degree can do. Law is the only good thing I can do with them, if not I can get into the careers it gets me in with just a portfolio.
I was thinking of resitting my A-levels next year if I didn't go to uni or decided I wanted to Chemistry and fully focus to get an A in a year and then maybe get a leeway into STEM or Pharmacology which isn't that competitive and pays better than the former. Or maybe develop a portfolio and do a course in UAL for Creative Direction which pays a lot.
I kinda had this delusion that once I got into a top uni like KCL/UCL I'd be fine or I'd change to a better degree like law or philosophy but I am kind of having a revelation that this won't work out. I don't want to go do a degree I moderately like only to have wasted a bunch of years and be poor after.

Okay, so at many universities - including those you've mentioned - you can't transfer into law from another course, because the competition for it is too high. Also, I'm pretty sure that even courses that allow you to switch require that you meet the entry requirements, so that's worth checking out. I think UCL and KCL both offer A*AA non-contextual for law and 3A's for philosophy. I'm not trying to dishearten you.
Also, if you don't want to do these degrees, then don't do them, that's completely fine. It's rarely advisable to do a degree you don't really like. I would, if I were you, have a think about what you enjoy and want to do. Even if the jobs pay quite a lot, you probably want to enjoy the degree and be able to stand the job and career sector.
Furthermore just on the note of Pharmacology, I'm pretty sure it will require 3 A's at quite a lot of places. It's not medicine, but I thought it was still fairly competitive.
Another note about uni applications and a-leveks which is worth checking out, is that some unis specify that you must sit three a-levels at the grades required in the same year to show academic rigour.
Have you already applied to the courses mentioned or are you looking to do so in clearing?

I'm eligible for contextual offer. I could just pick up Chemistry A-Level and study everyday to get an A in one year to show academic rigour, I could do this along side resitting my BBC if I wanted, I just need time (a year). I applied to them already and have an offer (UCL, KCL). I like the degrees I just feel like the career pathways aren't worth my time and money. I'm also worried about missing out on a university that is so good. Pharmacology offers ABB AAB for contextual students which I can get within a year. I just feel like I'm wasting time with no idea what I wanna do or what pays well.

When I said 'academic rigour', I meant that as a quote, that's the reasoning some unis give for requiring at least 3 to be sat in one sitting or so (not all unis require this, but it is soenthing to be aware of). Resitting would solve that issue if you do it in the same year, I believe. You may also want to check which unis discriminate against resits and for which courses.
I think if you aren't sure what you want to do, take another year out to figure it out. There's nothing wrong with that. Set up to resit if you think that's the right thing to do and try and figure out what course you want to do - if any.
Also, there's not really much point on being worried about missing out on offers from good unis if you don't want to study the courses they've offered you.

Do you think there's any point in doing law if I got BBB at A-Level? I'll basically be filtered out automatically, even if I got a degree from a RG or got a first

I can sit 3 exams in one sitting, that isn't a problem for me. I am just worried that me taking time out between my resit will disadvantage me more, e.g. doing a resit within a 2 year period instead of 1

No, it really won't. Some universities don't accept resits for law, like KCL, but the vast majority don't discriminate. The ones that won't discriminate don't care you did resits in 2 years not 1.