"While then British prime minister Tony Blair is seen in political circles as having “saved the monarchy” by urging the Palace to do more to show it cared after her death, his decision to describe Diana as “the queen of hearts” was seen by the royals as throwing petrol on the fire.
“The Queen has never forgiven Blair and nor have the others,” says one source close to senior figures in the royal household."
Now there are other reasons why Blair has not been given the Garter so far.First there is his age-he is still only 67(David Cameron & Theresa May will also have to wait). Secondly there are the people he has been associating with in his business namely those in Kazakstan, and the fact that he has not retired.
But this is a personal issue which does not relate to other ex PM's.Eventually I suppose it will happen-but things will happen very slowly