Try sticking X in the ICAO code box on the search screen and you will get a list of dummy ICAO codes that have been created to cover UK a/d that do not have their own code.
That doesn't work for me - it throws up a box saying that "Only EG locations are allowed".
Under PIB Help there is a document called UK AIS Additional Locations which gives a list of the X codes (like XLOC for Locking) but I can't seem to use these in the Departure or Destination boxes..
jb