One piece of tax relief a self-employed person gets is a reduction of their socialised health care tax: National Insurance. If you're self-employed you pay this tax at 9% rather than the 12% that employed people pay.
The government that commissioned this report tried to remove this particular bit of tax relief earlier this year, but was stopped as people were really annoyed about it.
We have this thing in the UK called 'Zero Hours contracts'. For example, an Uber or Deliveroo driver is on a contract where they can work as many hours as they like. The Uber driver is self-employed, and isn't an employee, so they have to do their own taxes etc.
The government claims that A) Zero-hours contracts are being used to provide less stable contracts for the employers benefit, as a zero-hours contract can be ended without notice (this is definitely true, though I'm not sure how many self-employed people are on zero-hours contracts), and B) that self-employment is overly incentivised in the tax system.
It sounds like this report will be used to raise taxes on all self-employed people because unscrupulous employers are abusing the tax reliefs. But this will have a detrimental effect on all self-employed people (small business owners, actual freelancers, software engineer contractors).