How To avoid double taxation with a job for Italy

Hello, I carried out a photography assignment for an Italian magazine. Apparently, I must attach a tax document from the UK to my invoice, or Italy will deduct 22.5% of taxes from it before it's paid. They don't know what tax documents are required, and I cannot find anything on HMRC. Any help would be greatly appreciated. Thank you!

Hi,
You will need to request a certficate of residence (How to apply for a certificate of residence to claim tax relief abroad) declaring your foreign self employment earnings.  
You would send the COR to the Italian tax authorities to claim back any tax deducted.  
You would declare the foreign self employment income in your tax return.
Thank you.