Apple - How can I make sure my iPhone uses UK spelling and keyboard?

You need to set your device language to English (U.K.), not just the keyboard. To do this:

1. Open Settings
2. Tap "General"
3. Tap "Language & Region"
4. Tap "iPhone Language"
5. Select "English (U.K.)"

Your iPhone should reboot and your dictionary should be set to British English. If it doesn't, you may need to reset your dictionary by:

1. Open Settings
2. Tap "General"
3. Scroll to the bottom and tap "Reset"
4. Tap "Reset Keyboard Dictionary"

You can verify that you are running your iPhone with British English as your preferred language as in the main Settings menu you will see an option labelled "Mobile" (it is labelled "Cellular" in US English).

In Settings → General → Language & Region → Preferred Language Order, you have to select English (U.K.) and remove others.

In Settings → General → Keyboard → Keyboards you can select the keyboards languages you want. There is a keyboard called English (UK).