I'd agree with taking maths as that can get you places, and if you're not sure then go for the core stuff and don't be specific. That way you'll avoid taking 'unfavourable' A levels and you won't be limited (except to sciences ofc). Marine biology and psychology I wouldn't recommend unless you're heading in a particular direction that you're sure of, or at least only take one. I want to do psychology at uni and I'm deliberately not taking the psychology A level because it doesn't cover the more scientific areas that I'm interested in - so you have to be careful and know what you're getting yourself into.
There's a reason why 3science+2maths is so popular. You don't have to take all of them, and you might want to add in one extra that you enjoy, but a straight maths/science mix will look very good, and I can assure you that so long as you choose the sciences you enjoy the most and work at them, you will get nearly anywhere you want to.