The US is one of the stricter countries when it comes to drug laws (the largest prison population in the world and more than 50% of inmates are in their for drugs), but to my knowledge when it comes to purchasing drugs through the dark web or probably online in general, they rarely if ever bust buyers. The CIA and/or other government operations are the ones who profit from the drug industry. The real reason drugs are illegal in general is because drugs are a trillion dollar industry and the various governments (powers that be) made drugs illegal to prevent competition... the lack of competition also allows them keep the selling point significantly higher. The various governments (especially the US), in particular the CIA, create and empower Cartels and other types of gangs to make independent manufacturers too afraid to compete. That's why cartels exist. The US gov could wipe out the Mexican/Colombian Cartels like a bug overnight using 50 year old technology, but obviously they're in bed with them and there's declassified proof of it.
For the most part I have found that they are typically more interested in busting sellers who are manufacturing their own drugs and competing with the good ole boys network, rather than buyers. In reference to the dark web, sellers are constantly getting busted, but not buyers.
Another option would be, in the unlikely event you were to get busted buying nembutal online. Always, deny the accusations, seek a jury trail, post bail if you have it and CTB (obviously using other means) in the meantime. But I'm not sure how that process works in the UK.