Eric Gill based his Gill Sans font on Johnston's Underground font. I think they're both good choices for accessibility, because their strokes are so thick, but I gather that Gill Sans is in wider use. Kestrell, I don't know whether you're embedding the font, or whether you have to use whatever is standard on people's computers.

(The font I use for my everyday reading is utterly without aesthetic value: Arial Rounded MT Bold. But it's very thick, and it's standard on Windows and iOS.)