Bottom Bracket (BB) shell tapping (for f**ked threads)

I thought facing was making sure that the faces (hence the assumption on my part) of both sides of the bottom bracket shell were parallel?

Likewise I had assumed that passing a tap through the threads of the bottom bracket to make sure they were cut correctly/empty of corroded on crap was to tap the bottom bracket?

What is re-tapping a bottom bracket?

Yup, that's facing, and tapping is (re-)cutting threads.
If the frame's any good, then it's probably worth getting the threads tapped when the current BB dies. A good shop or framebuilder will be able to do this.
If the threads are completely knackered, then there may not be enough metal left to cut new threads into. However, all is not lost - on steel frames (might work with ally too), a framebuilder can cut a slot in the BB shell, crush the edges of the cut, and braze it up again. This effectively reduces the internal diameter of the BB shell enough that the framebuilder can then re-cut new BB threads.