Although I'm not intimate with the BBC hardware this kind of thing doesn't necessarily indicate a hardware mode switch - the renderer could use double width pixels in the HUD simply to lower draw times(simpler rasterization loop) and conserve memory.

AFAIK really complex scanline tricks mostly appeared on the Atari and Amiga platforms first because they exposed explicit programmability that made it simple to define exactly what you wanted the memory layout to map to. Other hardware could do similar things, but needed to code a cycle-exact loop - a feat that was certainly known in the 80's, but also relied on a good understanding of the timings of all the hardware.