[di_canio_volley]

I would suspect the reason for the evacuation lies between these two possibilities:

At one end of the scale , perhaps a there was a threat to an (unspecified) pub or nightclub, a scenario which could kill hundereds inside the premises and injure a large number of people in the immediate surrounding area (as with the Bali bomb). If the actual pub or nightcub is not specified in the threat,then there would be no alternative than to evacuate the whole area.

At the other end of the scale, there would be the chemical or dirty bomb scenario. Perhaps this is less likely - if they possess such a thing, why didn't they use this on London on Thursday? (God forbid)

[Samwise]

I can't believe that nothing has leaked yet. Everything leaks.

[NO_2_CORZINE]

why didn't they use this on London on Thursday?...
Two Days of attacks causes more panic than just one day. One day followed by no attacks gives a greater sense of security. If they do it twice its the "Well they did it twice cant be that hard to do again"