What are the drug testing procedures for EU airlines?
I am not a pilot but you may find the following useful:
Civil Aviation Authority, INFORMATION NOTICE, Number: IN–2015/012

5 Drug and Alcohol Screening Programme
5.1 The  testing  process.
Trained  staff  should  carry  out  screening  tests  usually  by  mouth  swab,  breath,  blood,  hair   or  urine  sample.  Tests  are  becoming  simpler  to  administer  and  less  invasive  as  the  technology  continues  to  improve.  Minimal  notice  should  be  given  of  the  intention  to  undertake  random  testing.  The  sample  collection  and  testing  process  should  be  designed  to  ensure  the  result  is  reliable.  If  a  positive  result  is  found  further  (usually  blood)  testing should be undertaken. A confirmed positive result should be reported to the relevant Civil Aviation Authority regulatory authority; in the UK the report should be sent to the Medical Department of  UK CAA  for aviation licence holders.

It is easy to find companies whose business is testing UK pilots for drugs, for example AlphaBioLabs:

Protecting the aviation industry with drug and alcohol testing
We specialise in providing drug and alcohol testing to the aviation industry for both individuals who want to show that they are drug or alcohol-free and companies as part of their drug and alcohol testing policy.
Our aviation industry workplace testing consists of the following:

Random drug and alcohol testing
Pre-employment drug and alcohol screening
For-cause drug and alcohol screening
Post-incident drug and alcohol testing
Abstinence monitoring

We have a team of fully trained sample collectors in your area, ready to collect your drug and/or alcohol samples from your place of work or any safe environment.

...

Hair drug testing
Hair drug testing is a form of workplace drug testing that can be used to support other methods such as oral fluid and urine testing.
It can be particularly useful for a more comprehensive overview of a person’s drug use as it is known as a ‘wide-window’ form of testing, which can determine a history of drug intake for up to 12 months.

A different lab reports

British Airways, for example, tests urine for drugs and breath for alcohol. Workers can be randomly tested in the first six months of employment or if returning to work after rehabilitation, and can also be tested if it is suspected that drugs or alcohol may have caused an accident, or if they are suspected of being under the influence while working.

Relevant incidents might include things like this:

Air Astana, which is part-owned by UK firm BAE Systems, said it fired the pilot immediately and had a zero tolerance policy for drugs and alcohol.
In a statement to Reuters it said: “The airline regularly tests its crews for alcohol and drugs using a random sampling method.&quot;