In British English, both "practice" and "practise" are used: practise is used as a verb: "I need to practise more." practice is used as a noun: "I need more practice."

There is no difference. "practice" is the American spelling and "practise" is the British spelling. There are many different spelling rules for American and British English. In the 1800s Noah Webster, who wrote the first American English dictionary, wanted to standardize American spelling and to make it different from British English. So he changed the spelling rules a bit. For example, most words that end in "-or" in American English end in "-our" in British English, color vs. colour. Here is a list of the spelling rules: http://www.studyenglishtoday.net/british-american-spelling.html