If you wish to bring back wild plants, you will need to be sure that the plants are not endangered. Here the Convention on International Trade in Endangered Species (CITES) applies. These regulations are detailed and offer different levels of protection to species. Details are available from the Wildlife Licensing and Registration Service.
You can bring back CITES controlled plants which have been grown on a nursery, but you need to be able to produce documentation obtained from the supplier at the time of purchase. Orchids can be imported without permits only as cut flowers or where grown in flasks.
Wild plants, even if not covered by CITES, are often protected from collection in many countries. Increasingly the provisions of the Convention on Biological Diversity (CBD), which recognises a country's property rights over its native plants, will also come into force.
There are strict penalties for smuggling prohibited and restricted items. This can include unlimited fines, the possibility of imprisonment, or both.