The water from my bathroom tap comes out at an angle (as though I had my finger over the stream). The direction of the water changes as I turn the tap. It also makes squealing noises and sometimes sputters, making loud knocking noises when it does.
It behaves better if I run the shower at the same time. If I tighten it shut really hard, sometimes it will return to normal for a bit. I mostly stopped doing that though cos the seal broke and had to be replaced.
What’s wrong with it and how hard is it to fix? I have zero plumbing experience.

That style(Victorian) does not have an aerator in the spigot itself, so we can rule that part out.
You can, however still have a screen in the connectors of the supply lines.
Does it only happen on your cold tap on that sink or does the hot behave oddly as well?
That tap is very easy to rebuild. My guess at this point is that you have some corrosion or deposits in the faucet causing the behavior. If snaking something like a zip tie or wire into the mouth doesn’t result in removal of anything, I’d consider pulling the tap apart(after shutting the water off and releasing the pressure). There’s really only a couple rubber seals in it that are the consumables, the rest should be able to be reused after cleaning.
Only on the cold, though I almost never use the hot.
I’ll look at finding a wire or sth. I reckon I can take it from here. Thanks for your help.