I don't see anything ridiculous in somebody asking whether rapper might have got down to Cornwall or Kent during the 19th century. Apparently it didn't, but nevertheless, it could have done.

In A L Rowse's book "A Cornish Childhood", he recalls that many young men left his area to work in mines as far afield as South Africa, Australia, and America, and adds that some of the lucky ones came home with enough cash to retire on. So, while the mines of Durham and Northumberland were booming in the mid-19th century, a "Cousin Jack" might have gone up north for a while, learned a sword dance there, and brought it back with him. It's unlikely, but not impossible.   

Likewise, one of the many Geordie pitmen who went looking for work down south might have ended up on the Kentish coalfield and taught some of his new marras how to dance rapper. Unliklier things than that have happened - like a sword dance turning up on Papa Stour for instance.

Asking questions that go against the grain of received opinion is often the way that new discoveries are made. If our ancestors had always been happy to accept the official wisdom of their day without challenging it, we might still be living in the stone age.

Wassail!