As an ex-bus driver I can tell you that it is illegal to have push chairs blocking the aisle or the platform of a bus. Before the advent of low slung single deckers with sections for push chairs, all such had to be folded BEFORE a passenger was allowed to enter.
It may seem harsh to young mothers, but in case of an emergency it could be the difference between life and death for all on board.
The driver is also liable to lose his license if he allows any infringement of this rule.