Unknown white flower, looks like a huge spring onion

Unknown white flower, looks like a huge spring onion

Observation - Unknown white flower, looks like a huge spring onion - UK and Ireland. Description: Tennis ball sized. Just on the edge of the mud flats in the entrance to Shoreham harbour. Not sure if they're indigenous.

Tennis ball sized. Just on the edge of the mud flats in the entrance to Shoreham harbour. Not sure if they're indigenous.