Yes that is Star of Bethlehem (Ornithogalum umbellatum). There are 2 subspecies, one introduced (often grown in gardens) and one perhaps native in East Anglia. Whichever subspecies yours is, it is probably an introduction here. The introduced one (subsp. umbellatum) has larger flower and up to 20 of them. Your photo shows around 20 flowers and buds, but I can’t tell whether they are all on one stalk. The “probably native” one (subsp. campestre or angustifolium, depending which book you look in) has 4-12 flowers. I think these are badly recorded at subspecies level and Stace says that further study is needed, which slightly puts me off trying! If it is growing right on the river bank it may have arrived by river from a garden, as a bulb or seed.