Sword dancing I would think would be likely to be treated as within the category of "similar" to Morris Dancing. I'm quite sure that most people who aren't involved in folk music would regard it as essentially much the same.

I can't see how the fact that it takes place indoors woudl make any difference. Nothing about that in the amendment so far as I can see, one way or another.

Whether "similar to" could be stretched to cover Mumming is another matter. There are Mummers Plays where I think it could be. Especially if "dancing" is a matter of moving with musical accompaniment, as has been asserted by local councils on the basis of legal advice; the human voice is, of course, a musical instrument.