There are many different regional accents in both British and American English. The most important differences between American and British speech are as follows:
In some varieties of American English, certain nouns are pronounced through the nose and mouth at the same time. This isn’t quite so in British English.
British English has one more vowel than American English. This is the rounded short ‘o’ used in words like cot and dog. In American English, these words are pronounced either like the first vowel in father or like the vowel in caught.
In British English, r is only pronounced before a vowel sound. In American English, r is pronounced in all positions.
In many varieties of American English, t and d can sound the same. In British English, they have very different pronunciation.
Words like fertile, reptile and missile have different pronunciation in British and American English. In British English, reptile and fertile rhyme with ‘her tile’. In American English, reptile and fertile rhyme with turtle.
Words borrowed from French have different pronunciation in British and American English. In American English these words are pronounced with the stress falling on the final vowel.