is it cheaper over there ?Very little for a long time.
Mainly due to most farms being pretty extensive pastoral systems, and as such, didn't have a use for it.
The big kicker was the removal of gov't support and loss of markets in the 80's, which meant a dramatic shift in land use, and the value rose sharply. Possibly the big contrast was the response to WWII by your gov't as @holwellcourtfarm touched on, there was no encouragement here to grow cereals or more grass for that matter.
"sweat the assets" and there are few better ways to liquidate your fertility than grow hungrier plants and feed from the bag!
the big drop in usage for a lot of farms I know of came when it jumped from just over 100 quid/ton to 250, lots of folk still spent the same just had less fert, with more carful use grew just as much