Many species of mushroom contain psilocybin, a naturally occurring psychedelic prodrug (a compound which in and of itself is not psychoactive but breaks down in the body to become psilocin, the psychoactive substance).
Both fresh and dried psilocybin mushrooms and truffles, and innoculated mushroom grow kits, are currently illegal to possess in the UK. However, fresh truffles and grow kits are legal to possess in the Netherlands, and many Netherlands-based sellers will ship to the UK. Order truffles to the UK at your own risk, and please read up on responsible use.
Although innoculated psilocybin mushroom grow kits are illegal to possess in the UK, it is legal to purchase psilocybin mushroom spores for research purposes only. These can be combined with an (uninnoculated) mushroom grow kit to grow psilocybin mushrooms. This would be illegal if done in the UK.
UK sites selling uninnoculated grow kits and magic mushroom spores include:
Many species of cacti contain the psychedelic mescaline. The main genera are Echinopsis, of which the most psychoactive species is the San Pedro cactus (Echinopsis pachanoi, syn. Trichocereus pachanoi), and Lophophora, of which the most psychoactive species is peyote (Lophophora williamsii).
Mescaline-containing cacti are legal to possess in the UK. Preparing them for consumption, however, is illegal; this could include drying them, or cutting them into edible 'buttons'.
You can buy peyote and San Pedro cacti from: