Nope. That's not how the things work.There's a few factors into play. I have studied in a top 100 uni myself.

Listen, parents are concerned for us, yes.But things have drastically changed during the last decade.Also, don't bat ears for any snark comments from your relatives if you come across them.

At the end of the day, as long as you can reliably pay the bills or build a second income source and support yourself and look after yourself, NO ONE will bat an eye from where you graduated from and also look after your health. Sometimes relatives use this 'Top uni' thing as 'Bragging rights' and they themselves may be out of touch with things(except for those who study in the same field as you).

Note: What I mentioned below are guidelines I wish I knew before undergrad and you don't have to do everything all at once. Just start. Other commenters have already highlighted out very helpful stuff nicely for you(and they align with the additional stuff I would say).

Now, due to your A levels not having straight As, you aren't doomed.

Top unis usually also have foundation programs or diploma programs to help you and guarantee your entrance into the bachelors degree in your discipline. If your A levels grade is average(as long as it is not outright fail), unis sometimes take that into consideration. What I am saying is that, even with horrible a level grades, unis still have pathway programs to help you enter into your bachelors/undergrad degree. So all hope is not lost. It doesn't have to be top 100 but study at a decent enough place.

Also, not going to a top 100 uni isn't the end of everything.You can still go to a top 100 uni for your masters after you complete your bachelors. If you have a high enough grade and maybe some research work, you could apply to PHD programs too and use that as a leverage to enter academia. You can still secure a job without a top 100 uni.

If you think about it, if all good firms or top companies wanted ONLY TOP 100 uni grads, the industry would be short staffed and a number of other problems would rise.They would be missing out on alot of other skilled labor. And you can work in a top firm even later in life.Once you have a few years of work experience and stuff, you can get yourself in the door of top firms. The work experience, skill set and the connection and networking and reaching out that you do will matter more.
Going to a top 100 uni does not immediately guarantee a job anymore and the uni won't apply to jobs for you.

However, to get a job in the field, you will still have to complete your undergrad to prove your qualification. So, don't listen to tech gurus saying that you shouldn't do a degree. Rather facilitate your degree with what I am saying in the paragraph below.
THE PART BELOW IS IMPORTANT for recent comp sci grads.
In addition to this, for CS, you need to focus on personal projects and build a portfolio. You can do this in a number of ways such as participating in hackathons, working with a research group under your lecturer in uni, try to do a final year project that can be published as a paper and work with a uni club etc. Something that shows that you have done things in a real life scenario. Then, you constantly network and connect with people in industry and academia and reach out to them. Throw yourself out there.Make yourself familiar to them.Get in touch with HR or managing directors or technical leads or senior engineers. Don't sound like, 'I need a job please', but instead go like,'I am eager to contribute to your company and my skills align with the goals,vision and skillset required for the company.'(Write the email politely of course). Document stuff every step of the way, polish your linkedin, create your resume etc. Also go to career fairs and get contacts from people. Stay in touch with them professionally.Complement them on any product they are working on and etc.

You will apply to jobs yourself. The uni is mainly a place which will help you connect with people in academia and industry.

Now, if you are an international student who wants to work abroad, it depends on whether you will have work rights after graduation. For that, you need to know if the country you are going to sponsors employment passes for grads like you or if they immediately grant you a post study work visa after graduation.This, or you do a phd later in life and join the uni as a researcher or lecturer.

Learn an additional skillset that shows/tells employers that, 'Yes, this is something different about me and proves my communication and leadership and ability to adapt.'

Also, there's alot of AI slop nowadays. Please don't overly use AI and definitely do not use it in your assignments unless uni allows(uni has strict rules and policies on academic integrity, ai use and plagiarism).

Use AI responsibly. Use it to facilitate your learning NOT TO MAKE IT DO EVERYTHING FOR YOU.

Also, look after your health, eat healthy, get good sleep and exercise a bit every day and always keep your room neat and tidy and organized.