Don't know about british/american accounts .. it doesn't work that way.
but you can use both .. using external doesn't disable your internal drive functionality or anything like that. For example, I have a 250GB USB SSD on my Pro. I have a bunch of games copied on the internal drive and a bunch on the external. I can run any game from either any time I want to.
You can also choose which of the drives is selected as the default location for new game/demo downloads and installations, or copy things back and forth whenever you want to.