With time I've noticed that consistently British people don't express the letter "r" at the end of the sentences, while Americans do. Then I've checked it on Cambridge dictionary and I found that it is even documented not only one time. Let's take for example two words only: marble or certain.
In British English there is a full ignorance from the letter "r" which stands in the middle of the word (marble or certain), while the Americans express it well.
Now my question is if I can certainly determinate a rule which says that any word which have the letter R in the middle or in the end it is not pronounced at all in the British English. Can I rule it?