Alternate fields have an offset, but only if the horizontal sync timings make it so. If a machine is built to output a progressive image, there aren't really "fields", or you can consider them "all odd" or "all even" fields. I don't know anything about the BBC micro, but the Commodore 64 outputs a progressive image.
It's definitely interlaced output, though in most display modes the content is progressive (i.e., both fields look the same) - which is a bit of a dumb arrangement, really, since you get all of the flicker and none of the benefit.
Fortunately there's a command to switch it off, and no TV seems to mind displaying 50.08Hz output rather than 50Hz...
