Some associations are more thorough than others.

In England And Wales you can send a kitchen fitter on a 2 week course
to be a 'compotent person' under the England and Wales Building regs.

When I went to become a certifier of construction (to make it possible
for me to sign of my own building work in Scotland) i had to be
assessed by SELECT (The Electrical Contractors Assoc Of Scotland)

The assesment was very thorough, I was asked for my PASSPORT as proof
of ID, then my National Insurance Number. (they looked up the UK
register of electricians to check if I had any qualifications and if
they were up to date)

I had to do a few more courses to bring my regs upto date (did a regs
course 5 and a half years ago- had to do another as I needed one every
5 years)

The chap looked at 3 of my jobs with a fine tooth comb and asked lots
of electrical questions.

This is only to join the Electrical Contractors Assoc (of Scotland) -
SELECT.

The building regs are different up here but the ECA (in England and
Wales) has similar thorough membership criteria.

I also neede to prove insurances (public and employers liability) and
show my trade certificates.

Some associations have less thorough membership criteria, so it is
useful to check things first.

Some associations also have backup (if I go out of business they will
get a member company to take over ant arranged works at the agreed
price)