Why do the British add R's to the end of words that end with A?

The Americans tend to emphasise the "r" letter in any word by rolling their tongue. This letter is silent when the British pronounces words with this letter. Not sure why but I think the British way of pronounciation sounds more formal or regal. Just listen to how Simon Cowell talks as compared to Paula Abdul and Ryan Seacrest and you will know what I mean.