I've just finished second year at Cambridge, specialising in Soc Anth. I had no interest in Archaeology and still have never been on a dig. By all means go on a dig if that's what you're into, but there are plenty of people who haven't so don't feel that you need to. Don't bother with work experience unless you already have an idea what you want to do. Just read, do some travelling, go to some museums or whatever: do whatever interests you.
Re: A-levels- Biology will make first year Bio Anth easier, but it's by no means necessary if you're willing to work hard. History and Geography might give you essay writing skills but not really that useful in terms of content (maybe social geog could help you out in the human ecology bit of bio anth but most of it is common sense). By all means, have an idea of what countries exist in which continent and a vague idea what has happened in them- You'll find it a steep learning curve if you've no idea what happened in Bosnia for example.