What I think is cow parsley, Anthriscus sylvestris, which was growing in a small wood. That this is flowering in early February is an indication of the warmth of the winter we have been having.
Unfortunately, the photos I took of the plant in situ were rather blurred so here I only show photos of a specimen that I took home with me.
Here is a close-up of the flowers:
And of the leaves:
Specimen taken in Whiteknights Park, Reading, UK, on 2014-02-09.