Peyote
From RationalWiki
Peyote is a cactus button that contains the hallucinogenic chemical mescaline, and is an illegal drug with the exception of its sacramental use in Native American and South American religious rituals. It is also possible to use San Pedro cactus to get a similar effect.
You can buy it legally in the UK -- it is readily available in the London borough of Camden -- but they'll probably close this loophole soon, so get your arse in gear.
