Do any of these businesses have websites? Often there's a section, usually buried at the bottom, that explains the trading name, owners, registered address etc 'about us', 'corporate' or similar. Some smaller companies aren't necessarily registered with companies house, but larger ones should be.
DP works as a freelancer in various roles in the live events industry and yes, there is a lot wrong with the organisational side and I wouldn't be surprised if there was a level of tax evasion. I know that there certainly will be for some of the workers, but most of them probably don't earn enough to pay tax anyway.
Most frustrating aspects for him/us is that a lot of the work is done by volunteers who will literally work at an event in exchange for free entry, which makes it harder for people who want/need to be earning money for working, plus the volunteers aren't always as keen to actually work once they've got in and want to see the bands.
Plus the last minute aspect of booking people like DP to work. There are major festivals taking place in the summer of 2020 that have already sold out and he is likely to be working at, but he won't get a confirmed booking until weeks, or even days before the event, and this is for major essential projects that the festival cannot run without.