There doesn't have to be "trouble" before the police can use their powers to PREVENT a breach of the peace. (See case law involving the miner's strike, the 'kettling' of protesters and so on and so on and so forth)
The police spokesperson and the Accenture spokesperson both say that the group had been asked to leave AND refused. Presumably they then sat on chairs, ate pizzas and continued as though they were determined to remain.
The refusal was enough for the police to make a reasonable assumption that the group would have to be forcibly removed if they continued to refuse (and sit on chairs, eating pizzas).
Political rights activists KNOW this part of the common law and know how far they can go. In this instance, there was a positive result - no detention but maximum publicity.