I'm reading "Food in history" by Reay Tannahill where she discusses three crop rotation being introduced in the reign of Charlemagne, but I remember reading that this was only introduced to the British isles from 1750.
Is this a misrepresentation or were there particular conditions in England that made three crop rotation impossible until so much later?
The three-course rotation had been present for centuries, certainly being widespread by the 13th: I think you're getting it mixed up with the four-course rotation that took off from the 18th century, replacing the fallow with fodder crops that supported additional livestock and maintained soil fertility. England was in fact in the forefront of agricultural progress, readily adopting Continental innovations and by 1750 already a leader in arable productivity. The agricultural revolution was a long process, with early origins.