If you're hiring a worker and you have established they are classed as self employed for their engagement with you, you wouldnt be entering their information on any payroll software, employer records etc.
You would be paying them gross when they invoice you and they would be responsible for reporting self employed earnings.
Self assessment helpline could give them advice on what they are required to do.
I'm not sure what you're referring to when you say 'allowable expenses'.
I believe you may be referring to your self assessment reporting, again self assessment would be able to help you with this.
The most important thing I would stress here is it is your responsibility to ensure you hire the worker under the correct status - as employed or self employed.
There is a tool called CEST, you can use this to determine if the worker should be classed as employed or self employed for the engagement with you.
This can be found at:
Check employment status for tax