Hi Plums,
The female is registered with the BGS, but it sounds like she wasn't transferred into the ownership of the person who had her before you. I think the BGS would be happy enough to transfer her into your name, but you would need to wait on hearing back from them. I know Jane is a great new addition as BGS secretary, but given she has livestock of her own, including helping on the farm, I can't see her managing to work at the weekends as well!
Regarding the ear tags, there is no reason why you can't tag them now, and yes it is a legal requirement. Legally, the tags should have been put in before they left the old property. If the nanny is 5 she doesn't need an eartag, the tattoo is sufficient.
There isn't any great point is registering castrated males, however if the Saanen cross if female, you could apply to the BGS to register her, even if you don't know who her parents are. She would need to be registered in the BGS "Identification Register" which basically just says she exists. But then you can register her kids if or when she has any, and gradually breed them up into the Herd book.
Beth