Biggreen Universties have to stick with three A level offers because to not do so would disadvantage pupils at schools that do not do more than three, and they certainly would not disadvantage a pupil applying with only three from such a school. Of course some pupils applying with three will have proven their aptitude , particularly for medicine, by other means.
My DDs school normally encouraged pupils to aim for the better grades in three rather than stretching to four. However that was relaxed for Scientists because just three can really restrict their options. As with the OP Maths was a means to an end for my DD rather than her deepest "passion"
and she did not want to drop any of her other Sciences so four gave her that flexibility.
OP If you are going to specialise in the bioscience / neuroscience side of things I stand by Psychology as a good option at A level if that is where her interest lies . It means you can skip the basic modules at degree level and move on to the more advanced ones. Obviously if you want to study Physics fine, you can pick up the Psychology at uni, whereas the Physics routes at degree level would be closed down, but in terms of the knowledge you will need to be a bio /neuroscientist Psychology will be as, if not more, useful.
I would also add that in applying to courses do not assume that you need a 50/50 split in Biology / Chemistry modules either. There are areas of Chemistry that are extremely relevant for the biosciences and others, the "pure Chemistry" modules that are not. In reality the "soft" areas of Science study, communication, ethics, etc. are becoming ever more important as advances in the biosciences push into new areas. They can be equally valuable. I think there is a tendency for Scientists to think they have to go for the "hard" options, dare I say because of it's traditionally gendered nature. Psychology, ethics, communication might all be perceived as soft but actually are equally or more important depending on where a student potentially wants to specialise. In fact in terms of the knowledge you need to be at the cutting edge of the biosciences actually you could cast the net a lot wider than Physics or Psychology. The disciplinary skills being used stretch widely, obviously into engineering, but UCL even held a conference recently with games software developers seeking their input on how to manage the vast quantities of data that research is generating......