I'll share my knowledge of some of the british nasties, including commonly grown garden plants.
Aconite, containing aconitine, pseudaconitine, and quite a few other related alkaloids, extremely lethal, paralysing, induces cardiac arrythmia
Can be absorbed easily through skin, even from handling the cut plant or pats of it.
Aconitine blocks TTX-sensitive Na++ channels.
Hemlock (Conium Maculata) contains Coniine (2-propylpyridine), a toxic, volatile pyridine with a fairly sweaty, sour, fetid smell (as does the plants crushed leaves)
Acts as a non-depolarising neuromuscular blocking agent, and I just bet it can be absorbed through the skin quite easily, given its similarity to nicotine.
Hellebores:
Black hellebore (Helleborus niger) contains anaemonin type compounds, which act as irritants, some related species contain cardiotoxic glycosides, H.niger is common in gardens and as an escape.
Veratrum species (false hellebores) are extremely toxic, far more so than the Helleborus species, the root especially, containing the toxic steroidal alkaloids veratrine, veratridine, jervine, pseudojervine, and cyclopamine (which is a potent teratogen)
The veratrine type alkaloids open sodium channels (TTX-s type I think, although at a seperate binding site from tetrodotoxin), causing arrythmia and bradycardia, and a massive drop in BP, of some Veratrum species as little as half gram of root has caused fatality.
The jervines are interesting IMO, specifically cyclopamine, which blocks the activity of a gene called sonic hedgehog, involved in regulating morphological development of fetuses, and causes cyclopia, along with some pretty nasty mutations of the face.
Jervine itself is also found in some members of the genus Zigadenus (american, I am unaware of its presence in the UK) also known as death camas.
Water hemlock and hemlock water dropwort (Oenanthe Crocata and Circuta virosa contain polyacetylenic compounds called oenanthetoxin and circutoxin respectively, which act as noncompetitive antagonists (inverse agonists?) at GABAa, and circutoxin at least is a cholinergic neurotoxin, whos method of action specifically is unknown to me, these two plants are EXTREMELY lethal, and between the two of them, probably the deadliest native to britain.
Fungi of the genus Inocybe
Common, large group of basidiomycte fungi, most not larger than a few inches in diameter (cap), almost all are an identification nightmare, requiring microscopic analysis, usually small and brown, two or three species contain a quaternary tryptamine derivative, aruginescin, although almost all others are inedible and most are toxic, containing the quaternary ammonium cholinergic neurotoxin muscarine, and unlike in Amanita Muscaria, which bears traces of it, Inocybe species contain enough to kill, notably the red staining inocybe (I.patouilladii), I.napipes and a few others have caused deaths.
Fungi of the genus Amanita
Contain two, perhaps three main groups of toxins, discounting traces of muscarine here and there, and also discounting the orthosteric GABAa agonist muscimol.
Amatoxins (and the related phallotoxins and virotoxins, the latter found in conjunction with amatoxins in A.Virosa and most likely A.Verna, phallotoxins are inactive orally, although deadlier than cyanides if injected, in terms of weight potency)
These inhibit RNA polymerase type II, and target the liver, inhibiting protein synthesis and causing massive cellular damage, and exhibit a delayed action, typically 8-12 hours after poisoning do signs first appear, by which time damage is already done, amatoxins also undergo repeated enterohepatic reabsorption, recirculating the toxin in the intestine and sending it back to the liver for another round of destruction.
A.proxima and A.smithiana contain some funky aminoacid nephrotoxins with a rapid onset of activity.
There is also traces of a neurotoxin in A.pantherina, which also has been used recreationally like the fly agaric, which acts as a kainate/AMPA receptor binding excitotoxin
stizolobic and stizolobinic acids, these are active in very low ranges, incidentally I have read anecdotal reports of A.pantherina use causing long-lasting memory problems post use, I wonder if these are why.
Comparably structurally similar to KA selective excitotoxins from Clitocybe acromelalga, the acromelic acids, some of which are active in attogram quantities!! and cause long lasting post ingestion allodynia, and I just bet a lethal dose of acromelic acid(s) would be small enough, and odd enough, to be bloody difficult to trace forensically, and would cause months of suffering even if it didn't kill.
Some Entoloma (sinuatum, lividum in particular) contain as yet unknown gastrointestinal irritants, which may be fatal in some, E.sinuatum is known well to be really nasty, and has a reputation for a very rapid onset of action.
Cortinatius spesiosissimus, C.orellanus, possibly C.bolaris, contain orellanin, a bipyridyl nephrotoxin that causes seriously delayed kidney failure (by weeks or more, dose dependently)
toxic_plants2.pdf