The step nobody prepares you for is the one from working for yourself to running a business with people in it. On your own the sums fit in your head. The day you take on your first employee they stop fitting, and employers' liability insurance stops being optional.
Starting a plumbing or electrical business in the UK, and what you have to be registered for before you can sign your own work off: Gas Safe, Part P, a competent person scheme. Sole trader against limited company at your size. The insurance you are required to hold and the insurance everyone assumes is required. Growing with subcontractors instead of a fourth van, and what the business is worth if you ever sell it.