Well, you are talking about paragraphs. A single line of dialog is usually represented as a separate paragraph (in English at least). You would normally see something like:
Code:
<p>"But, I wan't space between," the OP said.</p>
<p>"Then us a paragraph," the replier said.</p>
<p>"Are you sure?"</p>
<p>"Yes, and if you want spacing between the paragraphs, use a style. And when someone comes along and tells you that this is a bad idea, just ignore him."</p>
That's what you would expect the code to look like in English. The styles for the p tag are used to set any space between the paragraphs and/or an indent.
Using a br tag to do this is generally considered to be bad code. There are plenty of examples using it, but, it isn't a good idea.
As to doing this, you should do it in the source document. The calibre editor is not really intended to write a book from scratch. Create the book in your preferred editor or word processor and then convert to epub. If you do this in Word, then it is a simple as pressing enter at the end of a paragraph. And calibre can convert this to an epub that matches the document within the constraints of how an epub works.