I've asked quite a few Londoners which longitude they live on and I've never been given an answer. It's one of those things which most people don't think about as they live their daily lives. If you prompt them with the word Greenwich, they do twig it.
They'll know the tube map off by heart though and they'll know that Poplar (where I was born) is E14.
E14 is an abstract way of expressing Latitude/Longitude. E14 0AR is a bit more precise. Food.Holds.Begins is even more precise and takes you right in to the front room of the flat I used to live in back in the 1960's.
Or, you could try to remember 51.508395,-0.013679
Postcodes are not a representation, abstract or otherwise, of latitude and longitude, they are a system for sorting and delivering mail.
The top of a mountain doesn't have a postcode, but it has a latitude and longitude.
The postal encoding system itself doesn't tell you
anything about the actual place, except in London where the letter part of the postcode prefix provides relative cardinal/ordinal directions from the centre, but the numbers are not meaningfully tied to the geography.
If you want to refer to a postcode in terms of coordinate systems, they can be described as an irregular polygon where each vertex has a latitude and a longitude. But, again, that's not very useful, since the postal region KA27 has an area of 432 km².