Site says last updated in 2011, but also explains the how and why:
Airfield Designators
All airfields have been given a specific identity code. If an airfield has a current ICAO code I have used this.
All other airfields (active or disused) have been assigned a code using a group that will avoid confusion with the official ICAO system. For these I have adopted a four letter alphanumeric code using the format Xnxx.
The first digit is X as this is not used as a first digit by ICAO.
The second digit of the code is a number which broadly indicates the location of the airfield. Numbers 1 to 5 are allocated to airfields in England by Latitude (51N to 55N), 6 is used for Scotland, 7 for Wales and 8 for Ireland. X9 codes are used primarily for private strips. X0 codes are slightly different, these being used for airfields that existed prior to WW2 only.
The last two digits are alphabetical with most showing two letters from the airfield name.
Examples of these codes are X4YR for York Rufforth (a disused WW2 airfield) and X0LH for the WW1 aerodrome at Larkhill.