My daughter has gone there for 5 years and she loves it. She is a mix of both sciences and arts. She'll be taking maths, further maths, physics (she wants to go into computing) and history (she's recently become incredibly passionate about it thanks to a teacher) for a level.
Regarding arts and humanities vs sciences, I can assure you that HBS is good for both. The school is big on drama, which my daughter loves and has taken for GCSE, and the yearly performances are great. They're even bigger on music, and I think on some technicality it's actually a music academy, as many girls who attend are already very high grades.
One downside is that drama is no longer offered as an A level (music, DT and art still are), but that was down to government cuts on state school and really not a reflection on the school.
There is little emphasis on testing, as the main source of pressure is from the parents from what I've seen, who've been dreaming of sending their girl to Oxford since she was 5. The school tries to combat this, so the atmosphere is quite relaxed, but the girls do put quite a lot of pressure on themselves.
There is a massive focus on English and History etc. The girls go to many performances of set texts, there are yearly poetry competitions, and the english teachers really love their subjects. As mentioned previously, my daughter who is quite mathsy has actually been swayed away from a fourth science a level due to an inspirational history teacher.
It is true that many girls do sciences, particularly medicine, but again I think this is because many girls come from families of doctors, or due to pressure from the home. The school tries to combat this as well, and at every school meeting I've gone to, the school has asked us to not try and pressure our girls into doing particular subjects, and repeatedly told girls to do subjects they love (because in all honesty this is what gets the girls higher grades, and the school better league table positions).
From what I know of my daughter's friends, there's an incredible range of a-level choices, and her best friend is doing the complete opposite of her: Art, French, English and Geography. If there is any pressure, I think it's swaying away from math/science. In her pshe lessons the girls declare their chosen subjects and the teacher always raises an eyebrow when a girl says she's doing something like Biology, Chemistry, Maths and Physics, and asks if that's what she really wants to do.
I can definitely say that the school encourages all subjects, and tells girls to do what they love, and that's probably why they're so high in the league tables!