"I felt that if we, as the Met, were not intervening once one person starts digging up Parliament Square, then someone else is going to join in and you have a spiral"

Michael Todd's quote is referring to the potential consequences of not intervening in a scenario. He is recommending that if the Met (Metropolitan Police) does not take action when one person starts to dig up Parliament Square, then it is likely that more people will participate in and the circumstance will spiral out of control. This might lead to more disturbance and mayhem, which the Met would then have to handle. Todd is stressing the importance of taking action early on in order to prevent a circumstance from escalating. He is recommending that if the Met does not intervene, then it might cause a much bigger problem that would be more difficult to handle. By stepping in early, the Met can assist to avoid a scenario from getting out of hand.