They will need a Home Office Licence - there is an annual music festival near me which the organiser spent last year having a media rant that they could not test "drugs" because the operator they chose did not have a licence for the site and dates. 

https://www.gov.uk/government/collections/drugs-licensing

Fact sheets & guidance including security, transport, standard operating procedures

Happy Reading.

A bit more detail based on experience.

Firstly, who are you? If you are the Crown i.e. the direct civil  service, then you do not require a licence(you still need to take precautions, but less paperwork involved). Similarly if you work directly for the police there are exemptions. Everybody else will require some sort of licence.

The type of licence will depend on the Schedule the drugs you are using. (Note Schedule not Class- Class is defined in the Misuse of Drugs Act and tells the court how to punish people found in illegal possession or dealing in the different types of Controlled  Drugs). The different Schedules (in the Misuse of Drugs Regulations ) describe the various security measures for working with different types of CDs. The licence is issued by the Home office and will include:

• How the drugs are to be stored
• Where they are to be stored( the licence applies to a particular building, not to the organisation as a whole)
• that there is  a system of recording use of the drugs- you record each use of the drugs and need to be able account for any drugs.
• And how you monitor the drugs and make sure they do not go missing.
• There is also a requirement for a  disposal protocol  for the drugs when you no longer require them. Typically this needs to be witnessed by a pharmacist or police officer.

An inspector from the Home Office will visit to check out you are doing everything as per the licence.