It’s important to remember that GCSE art, and all GCSEs are only building blocks towards something else - A-levels through to PhDs or apprenticeships, etc. They help teach you a few of the fundamentals and to engage you and help you think about where, if anywhere, you might want to take those things you are good at and which interest you. Nowadays GCSE’s in themselves don’t really lead into anything particular except the next rung on the learning ladder.
Just as with something like maths, if you choose to take it forwards you could end up being an accountant or a pilot or an astrophysicist. It sets some very basic foundations.
And foundations are really important. They open up options without shutting other things out.
It’s that thing they say about learning to become a circus clown, you first have to master all the other elements - juggling, tightrope, etc - before you then think about becoming a clown and looking at how to do those things wrong to make them look funny!
To be an interior designer, having a background in things like drawing, painting, art history, even sculpture and other areas is really important.
Post A-level the general art route is to a year’s foundation course. That’s when you start to think about direction. You (generally) spend the first term or so dipping your toe in different disciplines - painting, sculpture, design, fashion, set design, photography, illustration, etc. etc. - and then think about what area to focus on in the last part. That then can lead to a degree in that specialism - interior design or whatever.
There are other less academic routes (apprenticeships somewhere or just trying to start a career, etc) but they might be a harder thing to achieve as you would need good connections and would be competing in a crowded market against people better qualified. But even if you found an apprenticeship, they would almost certainly value that wider breadth of knowledge an art GCSE would bring over something that specialised so early.
My advice at GCSE level is to cast as wide a net as possible and keep options open, rather than looking towards the specific, especially focusing on a specific career.