Go to Settings > General > Keyboards > Add New Keyboard > English (U.K.).
Then remove any unwanted keyboards by tapping Edit on the upper right-hand corner.
If, instead, you mean the dictionary that’s used to give you the definition of highlighted words, then select (highlight) any word, anywhere on any text field. Tap the right arrow on the first toolbar that appears. Tap "Define" on the second toolbar. Then tap "Manage" on the lower left-hand corner.
On this screen, languages with no icon beside them are currently installed and in use for the “Define” feature. Those with a cloud icon are not currently installed, but you are able to install them at any time.
Tap the cloud icon by "British English" to download and install. Left-swipe on "American English” (and then tap the red Delete button) and on any other unwanted dictionaries to remove them. The cloud icon will re-appear, meaning that the dictionary has been removed from local storage, but can be added again from Apple's servers in the future, if needed. See illustration below.