It helped that a) I did Chemistry and Biology A-Level at a school that overtaught us beyond the syllabus b) my undergrad was at a psychology department that was focussed on the more empirical aspects of psychology, so had modules on Biological Psychology, Cognitive Psychology and 'Abnormal' Psychology, with some lectures on things like MRI and neuroscience. I now realise that is not standard for all degrees and my own undergraduate journey had little on qualitiative methods or the softer parts of psychology. That's the good thing about degrees though. Nothing stops you learning about those other areas if you devote the time to it and we had open lectures and seminars from other science departments.
The final thing is that doing my PhD was the time I learnt the most about neuroscience, MRI, pharma, neuropsychology and clinical disorders. Most of it I learned from my very hands-on supervisor on a 1 to 1 tutorial basis (which is what I do with my trainees, but that may vary from supervisor to supervisor). That is pretty much all I did for that portion of my life 6 days a week for 3-4 years and was paid to do that and write/teach about it.