[Talk-GB] Holes in modern England?

[Martin Wynne]

Anyone care to explain what's going on here:

 
https://geo.nls.uk/mapdata3/os/25_inch/holes_england/#holes_england_new/ol3

It displays the OSM basic map (without attribution), with some random 
blank patches (see for example a large area north-west of Oxford).

If you zoom in on the blank patches, they turn out to contain historic 
25"/mile mapping from the NLS.

Puzzled.

Martin.