On our recent visit to Devon we saw lots of these yellow flowers that look otherwise like the white cow parsley, wild carrot etc. A bit of web research suggests that this is wild parsnip.
Although the root of wild parsnip is edible, the shoots and leaves should be handled with caution as their sap contains photo-sensitive chemicals which can cause a reddening, blisters and burning of the skin. It is regarded as an invasive species in parts of North America.
It occurs to me that I have a similar looking umbellifer in the back garden, a fennel plant, but fennel does not have such poisonous attributes and flowers much later in the year.