There was a long thread recently that talked a lot about boarding for sixth form. Can't link as am on phone at mo, but do search on here for lots of people's opinions.
As had been said, there's really not much to choose between them when you have no preference over IB/A Level or location.
I'd say most of them are fairly rural, with access to nice small towns/cities. Thinking about Shrewsbury, Oakham, Kings etc.
They're all going to have excellent pools, art facilities and push the brightest pupils to achieve their potential in my opinion.
In terms of being in an all girls house - the amount of time spent with just girls is small. Part of that will be doing prep anyway. Lots of schools have societies going on in the evening, or pools and gyms open to use. As someone said, allowing boys in and vice versa will come doing to the school and sometimes the housemaster/mistress. Some schools have 6th form common rooms as well, so there's mixing opportunities there.
Few points I would consider:
I do think it matters how lenient the school is about going home for weekends, and how you feel about that. It really changes the feel when you get a lot of people leaving. To be honest I never really understand why students want to - the fun for me was always being surrounded by my friends, and exeats come round often enough! Although I've always been fortunate to be at / work at schools that make sure weekends are fun, so there's much more incentive to want to stay in.
Secondly, is DD bothered about being one of few new students or would she prefer to join a larger sixth form intake which mixes up friendship groups a bit more?
It's so hard when you're overseas, as really the best way is to visit. Often you'll find certain schools just click, and others won't.