I also would not agree with flying the flags at half mast if Blair were to die. He also did not have a great personal inspirational story behind him (unlike Mandela).
Thatcher and Blair dabbled in politics with varying degrees of success and deserve no flags at half mast. Mandela led an enslaved nation to freedom, he was the poster child for forgiveness and was a great personal inspiration to hundreds of millions around the world and for that reason the flags were shown at half mast.
To make a long story short, I could have cared less if they had the flags at half mast of any of them and I also would have cared less if they did it for all of them. But when it comes to who is a more deserving candidate for that honor, for me it is only Nelson Mandela.