Learn English – What does the idiom “heel to head” mean

"Taking the piss" simply means "making fun of" in its usual context. It's a very common expression within Britain, but probably rare elsewhere. ("Taking the mickey" is a similar, slightly sanitised version, that perhaps is used elsewhere?)

The Wikipedia page gives a fairly good overview.

I digress is relatively idiomatic, and thus, even though the present continuous would be normally used, i.e. "I am digressing" it isn't because it's been culturally solidified.

You mean the second sentence. The phrase is indicating self-consciousness of being off-topic, and therefore a return back to the original topic.