To answer my own question ... yes, you do lose the ability to search via postcodes with free maps.
If you select only the free map on a Garmin device (Tools \ Settings \ Map \ Info) then the 'Search Postcode' button is missing from 'Address', but if you select the 'CN Europe' map that came with the device then the 'Search Postcode' button re-appears.
That's been kept quiet...
So, I followed the instructions to create a list of POIs linked to all UK post codes and uploaded them. It's a messy process. The downloadable UK list contains errors (postcodes with no lat or long data). These lines have to be edited out. You also have to change the data format, and then the Garmin POI Loader app crashes if it sees more than about 500 different postcode entries with exactly the same coordinates, and it also crashes if you try and load more then 500k POIs at a time so you have to split the POI files up. I ended up with 4 files (500k max entries each).
To save everyone else the above hassle, you can find the set of postcode files I created for my Garmin at the link below. These are correct for August 2024. They load 1.7 million postcodes as individual POIs onto your device. You need to download 'POILoaderforWindows' from Garmin (free) and point the program at the folder with all 4 files while your Garmin is connected via USB.
[Please Register or Login to download file] To use on a device, you *don't* use the 'Address' funtion; WhereTo \ Extras \ Custom POIs \ All Categories \ Spell
Postcode is not case sensitive, but you do need to put the space in.
If you use the 'Spell Postal Code' button in 'Addresses' you're not using the postcodes loaded as POIs, but rather the postcodes that are embedded in the map data.
There is no means to put a street name or house number in, but a full postcode will get you to the street you want or one next to it.
My Garmin doesn't like displaying side street names for some reason even though it knows they're there (even with map detail set to 'Most') so if the full postcode gets you to the wrong street, just zoom in on your destination and tap the streets next to your destination to reveal their names. A workaround hack, but better than guessing.