[Update: The update is now rolling out in Poland too]
Samsung has now started rolling out One UI Beta update (Android 9.0 Pie) for Galaxy S9 and Galaxy S9+ in France, Spain, and UK. The One UI Beta program has already started in the US, Germany, and Korea.
The Software version is G960FXXU2ZRKL and G965FXXU2ZRKL for the Galaxy S9 and S9+ respectively. It has a big size, it is 1724.27MB.
Once signed up for the Beta program, you can check for the update manually by heading into the Settings > Software updates > Download updates manually. This will allow the software to update and load the beta version.
The stable Android Pie update for the Galaxy S9 and Galaxy S9+ is expected to follow in January 2019.