MEL is governed by EU Ops 1.085 when talking Flight Crew..........
The Commander shall: decide whether or not to accept an aeroplane with unserviceabilities allowed by the CDL or MEL.
So.....
As part of applying for an AOC an Operator will get approval to EASA Part M. Part of the Approval process is to submit a CAME ( Continuing Airworthiness Management Exposition). In the CAME will be the process of who and when certain actions can take place OUTSIDE of Part 145 which satisfy EASA Part M M.A.403
Maintenance. M.A.403 Aircraft defects
(a) Any aircraft defect that hazards seriously the flight safety shall be rectified before
further flight.
(b) Only the authorised certifying staff, according to points M.A.801(b)1, M.A.801(b)2,
M.A.801(c), M.A.801(d) or Annex II (Part-145) can decide, using M.A.401 maintenance
data, whether an aircraft defect hazards seriously the flight safety and therefore
decide when and which rectification action shall be taken before further flight and
which defect rectification can be deferred. However, this does not apply when:
1. the approved minimum equipment list as mandated by the competent authority
is used by the pilot;
or,
2. aircraft defects are defined as being acceptable by the competent authority.
(c) Any aircraft defect that would not hazard seriously the flight safety shall be rectified
as soon as practicable, after the date the aircraft defect was first identified and within
any limits specified in the maintenance data.