As for Multi-traders (croft) ... If you are good at what you do & are backed up by references + take great pride in your work then I don't see a problem with any of it of course.
I personally don't take a certificate of course completion as a positive thing straight away anymore unfortunately..
Only because a local multi-trader to me has his certificates plastered all over his van and tells me I am wrong when looking at a bouncing wooden floor a while back on a job.. when I pulled the floor up, the joists in question didn't meet the internal walls!!
I later learnt he has no work due to such a bad reputation :lol: