Really? I don't know anyone who is in that position who doesn't. Most pay themselves £12,500 salary, avoiding any tax and paying minimal NI. The rest is paid as dividends. Then they claim back tax on literally everything they can. Coffees in the morning, breakfast, dinner, lunch everything seems a taxable expense. Some even throw there non working wife in there from an additional £12,500 of tax free income.
Their argument is usually that they have no security in the work they do. But more often and not (from my experience) they have just as much and their day rate is 2 or 3 times that as an employee.
So use the £55k example the OP used. On £55k a PAYE emloyee would pay £5,064 NI per annum against £464 of a self employed.
Tax wise a PAYE employee would pay £9,496. Difficult to calculate the tax a self employed would pay but from my experience on £55k you would be looking at half of that.
I'm not an accountant and I'm sure I won't be right in all cases just speaking from my friends group point of view.