Qualifications improve your chances; they do not gate the job. Level 3 is the most recognised.
Level 3 most recognisedOpen entry
Your route in
You can start without qualifications - direct entry/volunteering; a qualification improves your chances
1
Qualifications employers recognise (Level 3)
Any of these strengthens your application.
2
Where this can take you
Level 2 foundation/intermediate building services then Refrigeration, Air Conditioning & Heat Pump Engineering Level 3; a valid F-Gas certificate is legally required to work with refrigerants.