Itinerant German musicians were actually not that unusual in Britain in the late 19th century, so it would be quite likely that the Bull and Bush would book such a band.

And the "tra-la-la" line is actually the start of Ach, du lieber Augustin, a well-known German tune, beloved of oompah-bands to this day, and many towns in Britain today have a Bierkeller with a band wearing Lederhosen