Why don’t British people speak English slowly and clearly to foreigners?
Because they have no idea which parts of their accent are understandable and which are not.
The truth is that most British people don’t have a lot of contact with foreign languages – they don’t learn them, so they don’t know how hard it can be to understand a native speaker! As a result, they do not modify their tone and voice to be understandable.
You might say: ‘’I don’t understand your English!’’, but this sentence makes absolutely no sense to them, because their English is THE English. They have absolutely no idea how to facilitate you since the way they speak is the only way they know.
Fey Lepura on Quora