Qualifications improve your chances; they do not gate the job. Level 2 is the most recognised.
Level 2 most recognisedOpen entry
Your route in
You can start without qualifications - direct entry/volunteering; a qualification improves your chances
1
Qualifications employers recognise (Level 2)
Any of these strengthens your application.
2
Where this can take you
Trainee entry needs no set qualifications; Level 2 building services installer is the recognised fitter standard, with Level 3 craftsperson beyond it.