In reply to nathan79:
I've just realized why maybe the Under Armour size chart appears to run smaller than normal UK stuff. (UA Med=30-32"whereas UK Med usually about 32-34")
They say the waist measurement is actually taken way up high at the narrowest part of your trunk, and not lower where you trousers waist normally sit. And this "true" waist is actually a few inches smaller than the waist measurement where the pants elastic band actually sits.
They also specify a hip measurement which maybe closer to the mark, but other manufacturers don't seem to use that.
If I go by their Hip measurement, I'm at the very bottom of their Sml range, not their Med.