Apium nodiflorum, commonly called Fool's-water-cress is a flowering plant found in ditches or streams and native to western Europe. It is a low-growing or prostrate perennial with pinnate leaves which have a vague resemblance to those of watercress

Watercress

Watercresses are fast-growing, aquatic or semi-aquatic, perennial plants native from Europe to central Asia, and one of the oldest known leaf vegetables consumed by human beings...

. Apium nodiflorum has short-stalked umbels of very small white 5-petalled flowers which are opposite the leaves and grow from the side of the stem at the leaf axils. It blooms in July and August.

It is not a poisonous plant, but it could be easily confused with the poisonous Lesser Water Parsnip

Berula

Berula is a cosmopolitan monotypic genus of flowering plant in the Apiaceae, containing the single species Berula erecta, which is known by the common name cutleaf water parsnip, or also simply water parsnip as are some other plants in Apiaceae such as Sium latifolium and Sium suave. It is easily...

– Berula erecta.
It is common throughout England, Wales and Ireland but is much less so in Scotland.