I have a 900A (AT&T version Note 3). I don't know where in the world it would be, but I changed the address of one of my contacts to be 1A5 B2C and it stayed that way, so I assume a real UK post code would work. It's just in the address block - not in any special "postal code" box. Are you using the default contact program, the orange "people" icon?
Having written many programs of this type, I'd say that a program that defaulted to ZIP code format on a device that may be used for addresses in other countries, would be sloppy thinking on the programmer's part.