It's not a difference in rate between employment and self-employment, though, and that's really the core of the issue. If you're self-employed your tax rates are (virtually) identical to those who are employed. If you set up a company and pay yourself a salary from that company, those tax rates are also virtually identical (although now you're having to pay employer NI contributions).
What's different is the tax rate on dividends from shares, and the tax on employment, and that's the tax loophole that IR35 seeks to close.
What's different is the tax rate on dividends from shares, and the tax on employment, and that's the tax loophole that IR35 seeks to close.