The Extent of the Cultivable Land by Professor C. B. Fawcett estimates that roughly half of the land is cultivable. For Britain, that means 26 million acres.

Agriculture in the United Kingdom 2020 by DEFRA states that the total UK cropland and grassland is 30 million acres, along with 12.6 million acres for grazing (see page 19, table 2.1).

Statistical Review of Northern Ireland Agriculture 2020 by DAERA states that the total NI cropland and grassland is 2.1 million acres, along with 0.3 million acres for grazing (see page 38, table 3.2).

Britain therefore has 28 million acres of cropland and grassland, along with 12 million acres for grazing.