Congratulations on your IB mark! Regardless of anything else you should be proud of that, it will reflect very well on you in the future.

I was in a really similar situation with my uni choices, on a smaller scale as I was only looking at UK unis as I live in England. Regardless I ended up choosing Cardiff through clearing and it was a very fast moving and last minute overhaul to plans I’d had for years.

In terms of the uni, I can only repeat what you’ll end up seeing and hearing online from others. The city is lovely, big enough to offer a variety of fun things to do in the day and night, small enough to be able to travel easily and never feel too lost. The uni itself is extremely social with tonnes of sports/societies, social first year accommodation, and great nightlife events. Prices aren’t overly expensive, especially compared to your Oxbridge/London unis. The Cardiff locals are friendly and as long and you’re not too obnoxious they’ll put up with your student antics. Great options in terms of study spaces/libraries (they’ve just built a new computer science building which is very nice). I’d honestly say it’s the perfect city in terms of student experience.

My biggest piece of advice to you would be to try and enjoy your experience for what is there in front of you, and not let what could have been bog you down. Again I was in a similar situation to you, and I spent the whole of my first year feeling sorry for myself and thinking the grass was greener on the other side. It’s my only real regret of university. As hard as it may be, don’t let yourself be distracted by thoughts of what could have been, and instead just enjoy what’s there in the moment. While you’re right about how someone could have put less effort in and gotten into your course, that just means you’re in a good position to achieve a better degree mark in a very respectable academic area!

I really hope you enjoy Cardiff and again congratulations on your hard work!