There's no difference as far as the Land Registry is concerned between parcels of agricultural land and parcels of land that just happen to have houses on them. They all have a unique registration number and are recorded on the LR database. So if one is subscriber to their system, you can get all the data the system holds and this includes the last price paid (if that figute is since about 2000, when they started doing this).
The trouble is often that the LR uses post-code to search for the system and postcodes for parcels of agricultural land don't necessarily exist. However if one know the land registration number it is always possible to find the post-2000 transfer price.
However LR don't provide this info for free, and it is only through commercial websites (Zoopla is certainly one) that land price transfers are available. Certainly Zoopla seem to limite themselves to land parcels with houses on them - and they provide the data sorted by post code.
So the answer is that the information is available (from the LR) - though not for free.