Hmm, interesting. What I'd actually done was responded to the question about speed limits in Street Complete by saying that there was no sign. Presumably, it combined this with information on lighting and road layout and knowledge of the UK to come up with this.

So, technically, if it is an unlit single carriageway, that probably is the speed limit, though you'd be crazy to try it!

This does seem at least misleading, feel free to alter this if you'd like to.