Always feeling directionless in life

No matter how much clarity I feel I have on something, a spanner is thrown in the works and I’m confused again.

I have never had a solid idea of what I want to do with my life and hate that the education system forces you to specialise at 16 essentially, as at the moment, a lot of careers I’m considering require maths or stem a levels.

If it makes you feel any better, you're definitely not the only one who feels this way. As a uni student, I can tell you, a lot of us, my friends and myself included, aren't 100% sure of what we're doing either. In fact, it's great that you already have some careers in mind! My advice, speak to people who are in the industry or working the jobs you're interested in to get a clearer idea of what pursuing that kind of career would entail. Mostly, all I can say is just to study things you're interested in, and it's important to know that it's absolutely okay to change your mind down the line! You may find that your degree can provide you with essential skills in what you previously believed to be unrelated fields. I can't tell you enough stories from adults I know who studied one thing, and ended up at the top of a completely different industry. At some point, you'll know what you want, just don't be afraid about the fact that that point may not be soon, because the journey in getting there is what is going to provide you with that clarity at the end of the day (hence why i switched courses last minute once i had a lightbulb moment about what i actually wanted to do loll)

No matter how much clarity I feel I have on something, a spanner is thrown in the works and I’m confused again.
Yeah, expect that a lot. It doesn't get any better further down the line.
Some people in their 40s are still trying to find out what they should be doing. If anything, you're definitely not alone.

I have never had a solid idea of what I want to do with my life and hate that the education system forces you to specialise at 16
Tell me about it. It's a drawback of the education system in this country, and the fact there is very little in terms of helping us decide or find this out. In fact, you would have to really go out of your way to find a way of finding out what you want to do.
The good news is, once you have found that something that really hits the spot, you can do it for life. The trick is finding it early enough to ennjoy a long career in it. Seneca once said "Life is long enough, and it has been given in sufficiently generous measure to allow the accomplishment of the very greatest things if the whole of it is well invested." (See: https://fs.blog/seneca-on-the-shortness-of-time/#:~:text=The%20Roman%20philosopher%20Seneca%20said,of%20it%20is%20well%20invested.)

as at the moment, a lot of careers I’m considering require maths or stem a levels.
That's a very good start. A lot of the required subjects tend to be in STEM, so if you have STEM subjects you can generally do a degree in most other subjects i.e. it definitely keeps your options open.
You're only limited to not doing music, some literature degrees, history, some geography degrees, some maths degrees (if you don't have FM), some art degrees, degrees with european languages, classics i.e. mostly things in humanities and some of the arts. If these subjects don't matter to you, then you're generally fine.

If you want some of the reflective questions that I looked into when I was picking my subjects for careers that I want, let me know.

The issue is I am already 21 and took 2 gap years. I also didn’t do STEM A Levels and i do not meet grade reqs for most stem foundation years.

Which specific subjects are you looking to doing?

There is at least one other alternative to A Levels and foundation years that I can think of, but I need to check what the subjects are before I give any recommendations.
With Maths, it's probably a better idea to just do the A Level privately; it's usually a lot easier than going into the other options.

Maths or comp sci, Bio, Chem
But maths is probably better than comp sci since it opens you up to more comp sci, tech and economics or finance degrees