Pilgrim, from what I have read there is no doubt Ultra did give at least 48 hours warning.
The suggestion is not what the RAF could have done, which was as you state, virtually nothing, but what other precautions could have been taken.
Actions could have included, moving more AA guns, searchlights and smokescreen machines into the area, temporary evacuation of many civilians, particularly the aged and the young, reinforcing the fire and ambulance services, and alerting the hospitals and civil defence teams.