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:
Both Bastiaan Braks photos and those from the Great Orme are ssp. cynapium
Fool's Parsley (Aethusa cynapium ssp. agrestis) A dwarf sub-species which is a rare [R] archaeophyte growing on arable land in South Britain and the Channel Islands. The longest flower stalks (pedicels) are usually shorter than the fruits and about as long as the bracteoles on the umbellets.
Fool's Parsley (Aethusa cynapium ssp. cynapium) is native and grows to 1m high, with the longest flower stalks are less than 1/2 as long as the bracteoles but about twice as long as the fruits. It grows in both cultivated and waste ground throughout the British Isles south of Central Scotland. This is much the most common sub-species of Fool's Parsley.
Fool's Parsley (Aethusa cynapium ssp. elata) The tallest at up to 2m. The stems lack grooves. Leaf-segments narrower than the other two ssp. Seen occasionally on Jersey, West Kent and Cambridgeshire. The least common, but might become established. These are better treated as varieties of ssp. cynapium.
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 which Hemlock does, namely the highly toxic Coniine (which is not a polyyne but rather an alkaloid [containing a heterocyclic nitrogen atom]). However, another source says that it contains Coniine-like alkaloids, rather than Coniine itself. Most of the literature on the supposed constituents of this plant are based upon a 1905 paper by Tutin when chemical analysis was in its infancy and apt to make many errors and omissions ! [A modern chemical analysis of Fool's Parsley is hidden by a pay-wall]!
Other toxic constituents of Fool's Parsley claimed by seemingly the same single source are
Aethusanol A/Ethusanol A,
Ethusanol B (none of which can 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'.