"Make no mistake, Scotland Yard was informed that Mr Blair would resign as Prime Minister if he was interviewed under caution.... They were placed in a very difficult position indeed."Difficult position is not the word, the Police should not be placed in a position like that ever, their job is to act according to political consideration, their job is to act according to the nature of a criminal investigation. A caution does not mean, necessarily, that you are a suspect anyway, the caution exists to ensure that the possibility of someone lying is made less likely.
The implication in the Telegraph story is that the investigation is waiting for him to step down voluntarily first before interviewing him under caution. If the story is correct, then the complaints about the length of time the investigation is taking actually reflects on Downing Street rather than the Met, but, simultaneously, undermines the position of the Police as an independent organisation that upholds the rule of law.
Imagine, if a non-political person found themselves a point of interest in an investigation and might have to resign if interviewed under caution. The Police would simply ignore a request to only be seen as a witness to avoid the embarrassment of having to quit. What is the point of the rule of law if its prosecution is constrained by political considerations? It completely undermines the notion that people are equal before the law.