MacBook for uni 2021

As long as it seems to be running reasonably well for web browsing, video content, document writing and spreadsheets, I don't see why you would need to buy a new laptop.

However, if you plan to use applications such as SolidWorks or MATLAB, they may run more slowly and will also take up considerable space. If you want to use Mathematica for light tasks, you could buy a raspberry pi 4 device (ranging from £30-£60) instead of buying a brand new laptop and just connect to it remotely as and when you need it:
https://blog.wolfram.com/2019/07/11/mathematica-12-available-on-the-new-raspberry-pi-4/

you can get more powerful computers than macbooks for the same money that'll last longer and won't be purposefully slowed down by apply during new updates (yes they do do that) if you wanted to buy a new laptop.

Your existing MacBook should be powerful enough for most tasks and if there's anything that required specific hardware/software you'll have access to systems that can run it, when I was doing my computer science degree there was a mix of laptops including some old MacBook Pros and airs because we all knew that anything intensive would be done in the labs or some had high spec gaming PCs at home they used, Im now doing a film degree and using my 2018 MBP for 4K editing and have no issues.

My suggestion would be to wait until you get to uni and see what you need but otherwise the new M1 MacBook Airs or Pros will be plenty powerful for virtually any workflow you need, you could even get an M1 Mac mini to keep at your flat and just use the air to take to lectures but I don't think you'll need it.