Any post office anywhere in the UK would see the "SE" and know the letter should be sent to London, however that happens.
Most people who live in Britain recognise the first part of postcodes for large cities and neighbouring areas. I'm sure post workers know all of them, they're mostly mnemonic.
Kids in gangs use postcodes to define their territory, they're that easy to use.
The small format is also seen on the back of many business letters as a return address.
