X_-_ codifications for UK aerodromes
On several sources, for example ourairports.com, I see several UK aerodromes identified by 4-character codes beginning with an X. Examples are X2BS for Binstead and X2BO for Bowldown Farm Strip.
Where do these codes come from? Are they still current?
Is there a comprehensive list of them - preferrably more or less up to date?