Here are some helpful spelling differences-- especially if you have to write papers in British English

• Words ending in an unstressed -or in American spelling end in -our for British. Examples are colour, flavour and honour.
• Words ending in an unstressed -er in American spelling end in -re for the British. Examples are centre and theatre.
• "Gray" is American, "grey" is British. However, those are basically considered interchangeable now.
• "Mommy" is American, while "mummy" is British. Also the same with "mom" and "mum."
• Cheque = check