OK, here's 2 calculations of scale.
Space Marines are 7' (at least).
Space Marine minis are 28mm (at least).
Therefore 28mm (game) = 7' (real).
So (divide by 7 to make the math easier) 4mm (game) = 1' or 300mm (real).
Dividing by 4, we see that 1mm (game) thus = 75mm (real).
So 1=75, in other words 1:75.
Guardsmen are 6' tall.
Guard minis are 1" tall (except they aren't, see previous post).
Therefore 1" (game) = 6' or 72" (real).
So 1=75, in other words 1:72.
So 40k is 1:72 or thereabouts - it's just really badly done. I find the closest commonly available scale is 1:35 (for kit, vehicles etc) - I use a fair quantity of old Tamiya stuff, especially on my orks, because 40k figures are so preposterously bulky. The early Citadel sculpts, and current LotR ranges are much more like 'truescale' 1:72.
Still Airfix weren't terribly consistant either, the 1:72 British Paratroopers were very different to the 1:72 Desert Rats, and the pilots of some of the aircraft (eg the JU88) were tiny. My suspicion is that many of the 1:72 Airfix figs (unless they've changed the sculpts since the '70s) are probably closer to 1:100 (1m=1cm, or 6'6"=2cm); and you may find that the '15mm' terrain is closer to 1:100 than 1:132 or whatever it's supposed to be.
You'll have to try it really - you never know, it may be OK!
:really quite a fan of Airfix honest cyclops:
"Well it's Forty-one Thousand Nine Hundred Sixty-nine OK -
Gotta war across the Milky Way - "
Iggius Popiscus and the Stoogii, "41,969"