Sonographers are trained from NHS healthcare professionals, such as nurses, radiographers etc. It's not something you go directly into as a graduate job.
Pharmacology is not a healthcare professions course, it's a science degree. It would be unsuitable for this aim. Biomedical sciences is also a science degree, not a healthcare professions course (although an IBMS accredited biomedical sciences course is the first step towards becoming a professional biomedical scientist, however as they are not clinical but lab based staff I doubt that would be a suitable background to become a sonographer).
Note that you don't, to my understanding, apply directly to sonography postgraduate degree courses. You need to be working in the NHS and apply to train as a sonographer internally and they will arrange for you to take the relevant course. I may be wrong about this but I've not heard of people just doing it separately and then applying to sonographer roles with no clinical experience whatsoever.