15mm is the scale used for Flames of War, Command Decision, Gear Kreig (WW2 sci-fi mecha pulp fiction), and other games. It's roughly 1/100 scale, thought I've seen 1/90 and 1/110 fudged in (just don't look too close). JR Miniatures makes some mighty fine terrain pieces, the old LFGS had boxes full of the stuff, fully painted. Fantastic for Command Decision games and the bigger ruin sets were excellent for Flames of War.
40k and WHFB are "28mm Heroic Scale", which has the figures features (particularly hands, faces and weaponry) exaggerated, often excessively so, out of proportion for "heroic effect". True 28mm is 1/56, but that's way too small for GW's chunky style, which is closest to 1/48 for terrain features, buildings and basic vehicle dimensions. 1/35 is generally too big, though GW weapons will still dwarf 1/35 and even 1/20 scaled guns. However, some 1/35 vehicles (particularly armored vehicles) and most artillery pieces can be bodged in. Just avoid anything that provides too many visual cues to how out of scale it is.