You're right we don't pay NI (at least not as much), when I was perm I think my NI was about 3k (+ change) a year.
Now it's closer to 300, however I'm paying around 20k more in tax overall than I ever did as a permie...
And my net pay isn't that much more (some months when I'm feeling frugal it's less).
What people continue to realise is that for insurance and liability we have to work through limited companies, which means OUR DAY RATE IS NOT OUR MONEY, of my day rate I pay out 19% immediately on corporation tax (what's the basic rate for a permie?) Then when I take the money out of the company and it becomes mine I pay personal income tax on it again.
Admittedly, this still works out less % wise than I'd pay as a permie, but £ wise HMRC are getting much more from me.
If I lost the ability to control who I work for, when I work or where I work I'd go back to permie, my income wouldn't change much but HMRCs take of my money would go down.