Hey, reading what you've written about what you want to do, I think going for a fashion design course is the better option out of fashion and costume. I'm applying to university later this year for costume whilst I'm currently doing a level 3 extended diploma in fashion, and comparing costume courses to the fashion ones my classmates are looking at has shown me that there is quite a big difference between the two.
Fashion design will give you a good ground level with both construction and design whilst allowing you to express your own design aesthetic, meaning you can play around with different styles throughout the course, and they will also help you develop your own personal image in fashion, so for pretty much the entire course you'll have 100% creative control, as they'll be teaching you how to become your own designer. In contrast, a lot of costume courses teach you how to work with different performances, people and companies, so they'll be teaching you specific skills that are more performance based such as historical garments, millinery and fabric distressing, which you'll have to learn how do even if you wouldn't use them in your own designs, as if you went to work for somewhere like Royal Shakespeare Company, you would have to do what the production required for you. Most costume courses do let you experiment more in your final year, but as you said, a large majority of the course will be designing for a brief as it's a desirable skill in the industry, and if you go to a university with a large performance department, they'll be getting you to design and make for the productions that are happening, as well as helping you develop your skills in the practical aspects such as quick changes and organisational skills, which wouldn't be as important in commercial fashion design.
Regarding jobs you can get afterwards, you can vary a lot with both degrees, from working for a large company such as New Look or the Royal Shakespeare Company to starting your own brand or custom costume business, but it can really vary on where you think your talents are and what companies are hiring at which time. However, most costume jobs will consider you if you have a fashion degree, so if you get to the end of your course and decide you want to work in costume, you'll still have a really strong education in a relevant area. I hope this helps in some way, even with my rambling
