Could anybody possibly help me finding the best font in InDesign (or another cc programme) for a "keep calm and carry on"-poster?
Actually, a bunch of Gill Sans MT typefaces are bundled with either Windows or Office. I suspect that is where the OP is sourcing them from. To be accurate, the fonts are not free but are licensed as part of the cost of the software with which they are bundled.
Still not get the sense and purpose of it
Before WWII posters were a major form of political and commercial communication. Britain was about to be overrun and everyone new it. Propaganda has moved on to the internet and posters in to condo livingrooms.