I fail to see the fascination with postcode. Whats wrong with town/road?
On the upside, TomTom in particular have dramatically improved their postcode accuracy in recent years, but than they had a poor base to start from (20% were incorrect when they first added the postcode function).
As to accuracy, portable units, including phones, are utter shite. Hence my last portable unit is somewhere towards the end of the Aston Expressway, when the piece of shit pissed me off for the last time. A proper inbuilt one can remain accurate even with no GPS signal for ages (days in the case of the CARiN fitted to MFL). The biggest reliability issue with Omega factory units is the ABS ECU... ...no speed pulse, no go anywhere.
Anyone choosing to disagree strongly needs to re-read and understand the technology.
That said, the portable/phone units are often adequate, as long as not in wooded areas or modern areas or large cities.