"Thank you for your enquiry to the Royal Horticultural Society's
Members’ Advisory Service.

Our Botanist Dawn Edwards says yes it is Allium triqutrum and yes you
wouldn't normally expect to see it flowering until spring. The plant
looks to be growing in a very sheltered position and we had a very mild
November December, which started lots of plants off early.

Allium triquetrum, a beautiful but rather weedy onion, native to
southern Europe. It is occasionally cultivated intentionally but
invariably becomes a nuisance, as it spreads freely by both bulbils and
seeds.

Allium triquetrum is closely allied to A. pendulinum, another
white-flowered species with a triangular stalk. The latter is distinct,
however, in having open, star-shaped flowers, petals each with three
veins not one, and in spreading only by seeds."

David @ the wet end of Swansea Bay