It's a free market so if you get your qualifications you can legally trade however the piece of paper will not tell you all you need to know to complete the most basic boiler service.
I am in the South of Ireland and see lots of N.I. reg vans driving over 100 miles to do oil boiler services for less than half what I charge, some call it competition I call it desperation.
I had four jobs booked in today, only did one the customer said he was using a lot of oil 1,000 litres per month last year he used 1,500 litres up to April.
Find the fault, fix it then deliver two lots of very bad news (my bill being one), he is now back to using the expected amount of oil for a 4 bed house.
Far as I know the N.I. market is flooded with oil service technicians, Phoenix are not helping with their special offers.
Have fun.