tiny white flowers in clusters
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Possibly, by the date, but it is always useful to take a picture of the leaves, as well, when IDing an umbellifer.
My Flickr photos...
could do with seeing more of the plant really. flowering in May, could well be Cow Parsley.
More detailed photos are needed, including photos of leaves and stem. Location info would be helpful, especially if observed near water. I'm not questioning your ID but more info is needed if your observation is to be helpful to others.
Lat/Lng: 50.8478, 0.205
OS grid ref: TQ553076