Current agricultural land use in the UK

This is interesting: Current agricultural land use in the UK savills.co 17 January 2019

The total agricultural area in the UK is around 17.6 million hectares, with an additional 3.2 million hectares covered by woodland and forests. The agricultural area, excluding woodland, accounts for 72% of the total area of land. The agricultural area has declined by around 26,000 hectares per year over the past 20 years. Reasons for this include transport infrastructure, building, woodland expansion (which has more than doubled over the past 20 years), nonagricultural use (golf courses, minerals, etc.) and some has been lost to the sea. [...] The current structure of agriculture in the UK is diverse, but it has changed significantly over the past 30 years. UK agriculture is on the cusp of an era of significant reform – what changes will the next 30 years bring? We also look at how land use might look in 2050 here.

("By 2050, 0.3 million hectares might be required for housing and infrastructure. This equates to 2.7% of UK land use and increases the current ‘built–on’ area by 21%").

So, at the moment, if responsible metal detectorists are staying off permanent grassland, 40% of the land surface is in fact out of bounds, only another 6% of the grassland is temporary.  Really, most of all those millions of finds should be coming out of just the 20% that is arable. Are they?