Is it actually a useful idea for navigation, though? I've never been sure, as it's rare that I need to specify a grid ref for actually navigating, and even more rarely to generate an 8-fig grid ref. The only time I really use grid refs is in making a route plan, and, in reality, I rarely use those... I don't navigate by grid ref; I navigate by associating the map with the real world around me.
Which brings us to the question: what are grid references useful for?
Grid references are only really useful if you're trying to exchange a position on a map with someone else. Or with something else (e.g. a mapping tool, or a GPS).
With the advent of computer mapping and route-planning, I can just click the mouse on a feature, and the tool should give me the grid ref very accurately, and then pass the route to a GPS receiver, if I want to follow a route. GetAMap provided by the OS presents mapping tiles that are 625pixels/km, thus giving a resolution of 1.6m per pixel. So, if I can point my cursor to an accuracy of one pixel, I can generate a grid ref accurate to 1.6m... Not that the mapping imagery is anywhere near that accurate, of course...