7% flat tax in Italy

I understand that my UK State Pension and any private pensions are taxable only in Italy, under article 18(1) of the reciprocal tax treaty between the UK and Italy: UK/Italy Double Taxation Convention. And that to ensure I do not pay tax in the UK, I need to complete the DT individual form and get it signed by the Italian Tax Authority. But does the UK/Italy Double Taxation Convention apply to the 7% tax as I have seen this 7% tax described as a 'substitute tax', to which the normal Double Taxation relief rules may not apply. Which would mean my UK private pension woiuld be taxed at 20% and still taxed in Italy at 7%?

Hi,
Please refer to the link below for guidance on which taxes are included:
DT10153 - Italy: Admissible taxes
Thank you