There's a few misconceptions on this thread. Firstly, there's not some unwritten preference for unis. Their preferences is what is written on the course pages. If they say they like course x, it does not mean that you have to do course y as well because it is similar to course x. If you are intent on doing neuroscience or any biomedical science for that matter, you don't need to do a 4th humanity subject or a 4th subject in general.
Maths is a useful A Level for sciences due to that bane of most math students' life -stats. It's used throughout the sciences for determining significance however, you do dabble in stats in other A Level sciences and you would also learn them at uni because everyone hasn't done the same courses so the course would also have to lay the foundations for this as well.
GCSEs have no correlation to A-Level, the step and change in style mean that whatever you got at GCSE has very little determining factors in predicting your successes in A Level or Higher Education.
For what courses you need to do at A-Level, look through this:
https://www.theuniguide.co.uk/search/course?utf8=✓&c%5Bq%5D=neuroscience If I were you I'd go with the consensus to leave the most amount of options open, so if 80% of courses required A Level x - take A Level x.
I don't really understand about keeping your options open because neuroscience is pretty specific so obviously there has been some thought there. why do you want to keep your options open for a more non-science related course?
The main thing to think about is, 'What grades am I going to get?' because it is the combination of courses
and grades which determines what unis you can get into. So if you don't think you are going to do well in a subject - don't take it. People's perception on how easy/hard a subject is shouldn't really have much weighting in your decision because everyone is different. I found Bio really easy, does that mean you will?
This has a bit of a ramble but thought it might help.
@Admit-One might be able to give a more qualified opinion
Thanks