When the IRA was in full offensive mode they would call in bombings to the authorities to give them enough time to evacuate areas before the bombs detonated. They wanted publicity & destruction but not injuries or deaths. By making the bombings more of a nuisance than a threat to life they figured they would avoid losing a lot of public support for their cause. They were against the British government, not British citizens.

Generally, someone involved in planting it would have phoned in to a newspaper and been like "I just planted a bomb at this place lol"