A worker’s employment status, that is whether they are employed or self-employed, is not a matter of choice. Whether someone is employed or self-employed depends upon the terms and conditions of the relevant engagement.
Many workers want to be self-employed because they will pay less tax, this calculator gives you a quick comparison between being employed, self employed or taking dividends in a limited company.
HMRC have a an employment status tool to help you determine whether a worker can be self-employed or should be an employee http://www.hmrc.gov.uk/calcs/esi.htm
If a worker should be an employee HMRC will seek to recover the employment taxes from the employer not the worker, so there are considerable risks for the employer if the status of its workers is wrongly assessed.
Some employers might decide to insist that sub-contractors must be limited companies, as companies can’t not be reclassified as employees.
The sub-contractor would then need to assess whether IR35 applies to their contract. If IR35 does apply then please read this blog on Deemed Payments