The higher the altitude, the thinner the air, giving less oxygen so the tree grows slower. This creates a denser, tighter wood than trees growing lower down. It's the same with Pacific/Oregon yew. England doesn't have any really high yew-rich mountain ranges. If we did, I'd have bought a chainsaw and some hiking boots.
There were a lot of very nice looking yew trees in Snowdonia when I last visited however...
You're being tongue in cheek with your comment of course, but it would be interesting to know if anybody has done tests on yew from various altitudes with density results. There's probably a table somewhere in a book... There may well be a point where the density spikes dramatically at a certain altitude where the conditions really slow the growth.