First of all, I am very happy to hear that you are interested in biomedical engineering! It's a great field, but then I may be a little biased since it's what I do.
Second, to address your question about whether biomedical engineering is very male dominated. Women are underrepresented in engineering fields in general, as you mention. This is a fact, not a stereotype: In the US in 2011, women received 18% of bachelor’s degrees, 23% of master’s degrees, and 22% of doctoral degrees (based on data published by the American Society of Engineering Education). I realize you are in London and the European numbers may be somewhat different, but probably not too substantially. However, as you can imagine, the distribution across different engineering specialties varies considerably. The highest percentage of bachelor's degrees awarded to women is in Environmental Engineering, with Biomedical Engineering coming in second. The percentage of women in biomedical engineering is around 40%, more than twice the overall engineering average. My own field, mechanical engineering, is around 12% female, which puts us below the overall average and close to the bottom. However, in biomechanics we definitely have a greater female representation than mechanical engineering as a whole.
In summary, I think you'll find biomedical engineering to have a healthy number of women students and lots of interesting opportunities. I encourage you to pursue the major.