Cross-bred goats can be registered with the British Goat Scoeity and in the show world are compeing on equal terms with the pure breds. Honestly even a "pure" British Toggenburg has some other breed in her, as that's how they were developed (from the pure Toggs), and you can take a BT, cross breed her to another breed and then breed back to BT for a few generations and you move back into "Pure" BT territory... all quite confusing.
If you want a milker that a) gives you reasonable amounts of milk, and b) is easy enough to handle for a beginner and/or chldren, then any crossbreed from Swiss-type stock would be ok - if coming from a reputable breeder and registered with the BGS (or pedigrees of the parent stock available). Maybe not too much British Alpine in her though - (British) Saanen and (British) Toggenburg Xes would be my ideal choice.
An older goat that has done it all a few times is also best to start with.
Golden Guernseys are very nice and placid milkers, with annual yields anywhere between 500 and 1200kgs per year, but they are a) rare and difficult to get hold of for pure-bred and registered stock, and b) quite expensive as they have become more popular thanks to certain TV personalities... It also seems there is a bit of a trend of leaving them in the "naural" state, i.e. with a good set of horns. Long-coated GG's require weekly brushing (preferably all year round). BUT they do have the "wow" factor if you take them out to a show