You absolutely don’t have to decide what career you want to pursue for the rest of your life when you’re applying to Uni. There are 3 key things that schools don’t make clear enough to 6th formers, but which my Uni tutor helped me understand during my first year when I was struggling with the same quandaries as yourself:
1. You can have more than one career in your life, and you can change paths at any point. Many successful people have worked across different sectors doing different jobs to get to where they are. Most jobs requiring a degree aren’t too picky on the subject, unless you’re required to be highly numerate or sciency.
2. Vocational subjects like medicine & law can be pursued at any stage of your life, and you don’t necessarily need an undergrad degree in those specific subjects to pursue them. With law, for example, it’s not uncommon for people to study a generic subject at Uni (from English to Biology) and then study a law conversion course, the GDL, prior to training to become a solicitor/barrister. This is because most people don’t get the chance to try out law before Uni, and are unsure of pursuing that as a career.
3. You can train after Uni for many careers/jobs with any degree. I had a friend who went on to develop a career in accountancy after having studied Environmental Biology at Uni, and his company funded his entire qualification without him even having studied maths beyond GCSEs. He’s very good at his job and earns a great salary.
You’re right to bear in mind that if you’re thinking about going into the sciences/engineering/medicine at some point in your career, a STEM degree will likely be ideal to get you there. And yes, some employers prefer humanities subjects, particularly if you wanted to pursue something like journalism where your writing skills will need to be high quality.
Most importantly, consider which subjects you enjoy at GCSE/A level. These will be the ones you excel at at Uni level, as the learning is largely all down to you and your motivation. Choosing law because someone has told you that you’d be good at it is really not a solid enough reason for choosing that course - you’ll need to be prepared for doing lots of reading of dry content and writing lengthy essays on similar matters. You may well be a great lawyer in the future, but you could entertain that thought later down the line if you’re not sure at the moment.