You Brits can't even search on the Internet without "Big Brother" watching what you're doing? That's like invasion of privacy. Don't you have similar laws to the US that protect you from illegal searches and seizures without a warrant, and that protect your privacy like the US 4th amendment?
Speaking in general, the government complex at GCHQ has, I believe, enormous abilities to view and screen electronic communications for security purposes. Certain trigger words or phrases do flag up particular observation, there's no secret about that (although the Triggers will vary according to intelligence received and they are confidential) . This function is, however, really focused on anti terrorism prevention and information obtained is shared with allies such as the US and European countries.
Whilst for very obvious reasons the exact scope of such observations is covered by the Official Secrets Act it's not very likely that our security services are particularly concerned about some private individuals attempting to obtain SN or similar substances for the purpose of ctb.
If it were to be flagged it would be more because whilst legal to purchase (I believe) it is a controlled substance under the Poisons Act and companies licensed to supply it are legally required to notify the Home Office of any "suspicious" purchases. That's why they only supply in certain quantities to registered business users or laboratories in most instances.