I couldn't tell you much about when they were introduced to Britain, but I
can say that in the Roman agronomists (Cato, Varro, Columella, 1st C BC- AD
1st C) there is a definite distinction in Latin between meadow (pratum) and
pasture (pascuus). There are also different types of meadow, ie dry and
watered meadows... The agronomists go into the different types of land in
great detail, and these texts have been discussed at length by K.D. White in
his seminal work on Roman farming.