"What maps were used by pilots in WWII?"
The Ordnance Survey produced an RAF edition pre-War (they also produced a civil aviation series pre-War) based on their quarter-inch maps of the UK - quarter-inch is 1:253,440, so almost quarter-million - and IIRC a half-million series. These became the RAF Edition (War).
Much of the RAF's mapping was produced by the Geographical Section of the General Staff and there were dozens of GSGS series at different scales and in some cases several editions of particular series. Some series covered the UK but most were overseas; GSGS produced map series that covered much of the world, although maps in overseas theatres were also produced by bodies such as the Survey of India.
The Foldex maps were apparently produced for use by single-seat fighter pilots, being cunningly folded to enable them to cover long distances without having to wrestle with large maps in a confined cockpit.
GSGS also produced the plotting charts used by navigators - bare outlines with little geographical detail, although ranges of mountains were marked, but with heights in metres which seems to be asking for trouble, given that altimeters read in feet!