Restricted goods - personal customers: http://www.royalmail.com/personal/help-and-support/Tell-me-about-Restricted-Goods
Items that can be posted in UK post but with restrictions.
"Living creatures, insects and invertebrates: Including bees, caterpillars, cockroaches, crickets, destroyers of noxious pests, earthworms, fish fry and eggs, leeches and other parasites, lugworms, maggots, mealworms, pupae and chrysalides, rag worms, silkworms, spiders and stick insects.
Must be boxed and packaged to protect the creatures, our staff and our customers from harm. Use 1st Class as the minimum service. Items must be clearly marked ‘URGENT – LIVING CREATURES - HANDLE WITH CARE’. The sender’s name and address must be clearly visible on the outer packaging.
Dead insects, sent as collectables, are also allowed."
I could find no mention of fungal or plant material although pathogens (presumably human pathogens?) are prohibited in the Dangerous Goods leaflet.