Language is never fixed. It changes over time as people try to express themselves in different ways to different people. And English, although broadly the same language, is spoken in lots of different ways by lots of different people. Normally this is because people live far apart. However, even in Britain, a relatively small place, there are huge variations in the way people speak English. And that is not to forget the large numbers of people who speak other languages as their first language - people who have migrated to Britain from abroad, as well as the significant 25% of Welsh speakers who still speak Welsh in their local communities.
Written English is much more fixed, and changes much less quickly, than spoken English. People across Britain speak English using not only different accents, but different expressions. For example, people of different ages and from different parts of Britain may greet you with as many different words as; dear, love, darling, chuck, mate, guv, son, sir, madam, miss, fella, and many, many more. As people migrate into Britain, and as people move around Britain more freely, accents change more quickly. However, most British people can guess where another British person is from because of their accent.
One of the most obvious differences in accents in Britain is shown by the pronunciation of the word 'bath'. Broadly speaking people in the north pronounce the 'a' as if they were saying 'at', and people in the south pronounce it as if they were saying 'are'. The same distinction is true of lots of words which have a similar sound - like 'laugh', 'ask' and 'dance'. There are other vowels that are pronounced differently in the north and south - including the vowel sounds in 'cup', 'rose', and 'bird'. These differences are harder to explain than the 'bath' example. However, although different accents may sound difficult at first, they shouldn't be too hard to understand once you get used to them! .
The same can't be said for different areas which use completely different words. For example, the 'Cockney' speech, which describes a small area of east London, is nearly impossible to understand for anyone else. This is not because of the accent, but because they often use rhymes to describe things instead of the words themselves. This originally started in the 19th Century as an attempt to make sure that the police could not understand what they were saying if they were doing something criminal, though it is not the same now!
I'll leave you with a few fun examples of Cockney:
'Adam and Eve' (meaning: believe) - E.g. 'I don't Adam and Eve it', means 'I don't believe it'.
'Bacon and Eggs' (meaning: legs) - E.g. 'Move your bacon and eggs', means 'move your legs'.
'Pork pies' (meaning: lies) - E.g. 'He's telling pork pies', means 'he's telling lies'.