How to turn off/where is button for International CDMA on iPhone SE?

To turn off: Hold the SIDE and VOLUME UP buttons until slide to power off appears, and do so.

To turn back on hold the SIDE button until an Apple logo appears.

CDMA phones are sold connected to a specific carrier. There is no such thing as International CDMA. If you call the carrier you want to use and tell them the MEID from Settings/General/About they may be able to link your phone to their network. But most CDMA networks have been shut down or will be shortly.

What are you talking about?

No iPhone has ever supported CDMA outside of the US, and the US CDMA networks are either already shut down or will be by the end of the year.

You can not use an iPhone on a non-US CDMA network.

That option no longer exists, and even if it did, it would serve no purpose. There are no CDMA networks in the UK.