Outside the USA, writers frequently use US English when other forms of English are more appropriate.
Set up your computer to use the correct form of English. Doing so affects the proofing tools (spelling and grammar checkers). If you do not do this, Word’s spell checker will fail to pick up cases of American spelling, leaving your document with spelling errors.
In this article, I take British English to be the desired form. Some examples of spelling differences:

·         Colour (color in US English)

·         Flavour (flavor)

·         Honour (honor)

·         Metre (meter)

·         Enrol (enroll)

·         Fulfilment (fulfillment)

·         Travelling (traveling)

US English uses z, not s, in forms such as realise and realisation. With British English, both forms are acceptable (realise and realize), although the s spelling has become the most commonly used form.
British English differentiates between nouns and verbs with words such as licence (a noun) and license (a verb). US English does not do this.
In US English, programme is always spelled program but in British English the spelling is programme, unless it refers to computer software (in which case the spelling is program).
Collins lists nine differences between the two forms.
How to change your English form. The process is simple and takes just a few minutes.