I'm about to start my third year of my biological science degree and am interested in a similar area and have done a lot of research so hopefully what I've found can help!
First thing with a Biomed degree is that by the end of it you mostly likely won't offically be a qualified "Biomedical scientist" in the UK, as normally you need a masters to achieve this (heard from biomed friends and lecturers). A way of getting the master is the Scientist Training Program (run by the NHS, where you spend 3 years working for the NHS and earning a masters at the same time) or just consulting the board of biomed for the UK for another masters program. However, biomed is a good way to get into toxicology as said above. Pharmacy is always a good shout but is also a longer degree than biomed.
I might be biased but biological sciences is good for toxicology but is a lot more broad so you probably wouldn't be able to 100% focus on it for your degree, and it'd be less clinical based. I do think bio is good if you're indecisive about what you want to do though and if you think you may change your mind on your main interest (I know I did).
Overall for toxicology, most life sciences are good. Do a google and see what toxicology masters are asking for to have an idea! I found King's Colleges want biochemistry, pharmacy, or a related subject.