Need help with the complicated stringent laws of working in Denmark

Hello Everybody

I am a new member here. However I am not so new to Denmark. I have been working and living here for 3.5 years. I am British and working in the Material Science & Engineering division of VELUX.  I live just outside Aarhus in Risskov and loving it. 

I do have a need for some help with the complicated stringent laws of working in Denmark, not to mention BREXIT confusion .  As far as I understand before going to any authorities right now then I still have most rights as I did in the EU rules. Not that I really can find out what they are either.

Anyway, I wonder if there is anyone that could help me in what the rules are if I take on an additional part time role in my spare time as a Yoga instructor. I have been asked to teach at a local physiotherapists so essentially it would be a paid role but not exactly that much maybe 3-5 hrs. Anyway the point is do I need to have an additional work permit for this?  Also, I would like to understand for future if I can become a self employed yoga instructor again still with my day job so just as a supplement as it is more of a nice hobby job. I have no idea how to do that, since it is not going to be my main line of income so I'm thinking I don't exactly need a full on business plan.  Can someone help me here if they have done something similar?