Can you tell the difference(s) between undergraduate and pre-university? Or is it the same thing? I need to know the difference in helping me to choose schools/universities in UK for my A-Level study next year...

Pre-University is a course you may take for like a year or so as a path to getting to university, but generally not needed if you get enough UCAS points.

Undergrad is a normal university degree - e.g I am a first year, so I'm an undergraduate (because by definition, I haven't graduated yet)

Postgrad is a post-degree course like a masters or a PHD


choose schools/universities in UK for my A-Level study next year

You don't do A levels at university, you do them at a college or school sixth form... What A level grades you get determines how a good a uni you can get into for a degree course