Some similarities to : Wild Carrot (Daucus carota) but that has longer bracts (and no bracteoles) and they are pinnately divided.
There are 3 sub-species:
Your Author thinks the above photos are all of ssp. agrestis.
Fool's Parsley (Aethusa cynapium subsp. agrestis) Native, found on arable and waste ground. Erect stems to 1m high. The longest flower stalks are usually <half the length of the bracteoles. The commonest Fool's Parsley growing mainly over much of the UK south of Clitheroe.
Fool's Parsley (Aethusa cynapium subsp. cynapium) Archaeophyte. Stems only to 20cm. The longest flower stalks are about as long as the bracteoles. Grows in arable land, scattered in South Britain and the Channel Islands.
Fool's Parsley (Aethusa cynapium subsp. elata) The tallest at up to 2m. Lacks grooved stems. Leaf-segments narrower than the other two ssp. Seen occasionally on Jersey, West Kent and Cambridgeshire. The least common, but might become established.
Fool's Parsley derives its name from being mis-recognised as
Parsley and eaten. Being toxic this is not a good thing to do...
The plant has apparently been used in folk medicine, but this is not recommended with a plant which is so toxic. On ingestion, Fool's Parsley can cause excitement followed by depression leading on to skeletal muscle paralysis, vomiting, diarrhoea, dilation of pupils, and finally death by suffocation. It seems not to affect the heart.
It is said (seemingly derived from just one source) that it contains the polyacetylene (aka polyyne)
Aethusin which has 2 triple bonds. Polyynes are toxic, and this one is no exception. The same source implies by proximity of the drawings, that it also contains the polyyne called
Panacaxacol, which is a longer molecule with several extra side-groups attached but still with only 2 triple bonds. Fool's Parsley is said to be not as toxic as Hemlock is, but it is only just below it in toxicity. It is also claimed to contain the same toxic alkaloid as that, the highly toxic Coniine (which is not a polyyne but rather an alkaloid [containing s heterocyclic nitrogen atom]). However, another source says that it contains Coniine-like alkaloids, rather than Coniine itself.
Other toxic constituents of Fool's Parsley claimed by seemingly the same single source are
Aethusanol A/Ethusanol A,
Ethusanol B (none of which which your Author can find any chemical structural formulae for - but they might have synonyms which are findable, if only your Author could find any synonyms for them).
All in all the constituents of Fool's Parsley are calling out for a more recent evaluation of the secondary metabolites, preferably from specimens which have been verified by a botanist, since Umbellifers are so notoriously self-similar when you have not 'got your eye in'.