The definition is "not 100% exact" in its nature, but dealers have been prosecuted and sent to prison for violating this.
It's also possible to apply broad bans to Markush structures (https://en.wikipedia.org/wiki/Structural_scheduling_of_synth...), which is how I understand some places attempt to regulate synthetic cannabinoids and other broad classes of chemicals.
(Keep in mind these laws do not apply to fentanyl -- in the United States, fentanyl is schedule II and is available as prescription medicine. Illicit fentanyl is more "illegal drug market" than "research chemicals"; cocaine is in the same boat in the US as well, a schedule II drug that's available on the illicit market. The above might apply to any fentanyl analogues though.)
http://www.legislation.gov.uk/ukpga/2016/2/contents/enacted
https://en.wikipedia.org/wiki/Psychoactive_Substances_Act_20...
But also the UK has rapid powers to add a substance to the schedule in the Misuse of Drugs Act. Here's a recent list: https://www.gov.uk/government/publications/controlled-drugs-...
Here's schedule 2. I don't know how often this gets updated: http://www.legislation.gov.uk/ukpga/1971/38/schedule/2
> You can't legislate a state of mind,
You can legislate "intoxication", although the methods for doing so are pretty crude at the moment.
The Psychoactive Substances Act 2013 was introduced in NZ after its failed experiment with legalising synthetic highs. I don't necessarily agree with the act, but it basically is a "broad ban" of new recreational drugs.
https://en.wikipedia.org/wiki/Serotonin_receptor_agonist#5-H...
It would likely also lead to unforeseen consequences. For example, the Czech government attempted to ban growing Phalaris, but had to backpedal as it's a common unassuming weed.