IPhone – Change to a second language dictionary while readinjg

It looks like this is possible in iOS 7. When you click to define a word, in the bottom left there is a "Manage" button. Here, you can tap the X for a dictionary that you want to delete from your device and tap the cloud button to download a dictionary you want. So in your case, tap the X to delete the NOAD from your iPhone, and tap the cloud to download the OED to your iPhone.

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).