The good thing about Grammarly is once you have select British English on one device is will remember that setting on all devices because it’s an account setting change, so first make sure your logged in.
In Browser Extension
Click the Grammarly icon in the top right of your browser, then in the setting “I write in” choose British English.
In the Editor
Navigate to your document dashboard and click “profile” on the left. Click the “Language Preference” tab and then select “British English“.
Mobile Keyboard App
Go to “Grammarly Settings“, Click “Language Preference” and choose “British English“.
As time moves on and so does the design of Grammarly, we will keep you updated as to where the preferences are.