Actually if you read the whole Livingstone statement he did recognise that the kind of people involved in organising these kind of atrocities are often not afraid to die in the process of carrying them out -
"Finally, I wish to speak directly to those who came to London today to take life. I know that you personally do not fear giving up your own life in order to take others - that is why you are so dangerous."

But in fact I think it likely that Livingstione would have been making a reasonable working assumption that, on this particular occasion, these weren't cases of "suicide bombing" ,any more than they had been in the Madrid train bombings.   

It may yet turn out that this was a mistaken assumption, but I'd suspect that the people organising the bombings would wish to conserve their limited human resources, and would be more likely to use timing devices where possible. That appears to be how the London police are reading it too at present.

And I still think it was a pretty good statement, especially in the way it set out to derail attempts of racists to exploit the bombings. It's worth clicking on that link I gave and reading it in full.