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?

X codes in general are unallocated by IATA and reserved for non-international internal use by individual nations. Given the limited number of 2 letter codes in the EGxx range it makes sense to use X codes for farm strips. X2 makes sense because G and 2 are both allocated to the UK for radio callsigns.