An additional format not in you list is LLDL (WC1N).

The two parts form the incode (which postoffice) and the outcode (street address). The outcode is always DLL. The incode can be LD, LDD, LLD, LLDD or LLDL.

You can use ZZ99 9ZZ for an official "no postcode" and I think ZZ98 9ZZ also can be used for special purposes.