I don't have a preferred subject listed by my dream university course.

I desperately want to study marine biology at Queen's University Belfast.
It says "BBB including Biology and at least one from Chemistry (preferred), Geography, Mathematics or Physics". There's also a mention of "ABB including Biology + GCSE Chemistry grade C/4" so not even another science related subject, which gave me some hope - but is it false?
I haven't chosen my A-level subjects officially yet, but I don't think I'd manage, and definitely wouldn't enjoy, chemistry.
I'd like to take biology, geography and psychology at A-level, which also meets the requirements of the other courses I've looked at - some even count psychology a second science, but I could love no where else as much. Some people have told me I won't have very good odds of getting in, and I'm honestly devastated and terrified. I've considered doing maths or physics instead of psychology to up my chances, but even if I could achieve a good enough grade, I'd slack on the others which are more important, so I've concluded that it's really not worth it.

You need to think more strategically. Preferred subjects aren't just picked at random, they are related to the degree and career after. If you don't like the subjects at A level, or find them boring or hard, then you can pretty much guarantee that you won't enjoy the degree, or the career. So having a 'dream degree' is just daft if you can't see yourself doing the ideal A levels, or getting competitive grades.