There is no "new " tax. There are changes to existing taxes. If you are yet a temporary (migration) resident in Argentina, you will not be liable for any taxes until you have become one and then completed twelve months of temporary residency. Then, if you are from the UK, you may find you are protected from paying tax on the pensions you mention by article 18 of the UK Argentina double taxation agreement:
https://assets.publishing.service.g...data/file/498342/argentina-dtc_-_in_force.pdf
Those of us who believe we are in the happy position of having double taxation treaty protection have to make a decision at the point we become taxpayers in Argentina (i.e., at the point we complete 12 months of temporary (migration) residency): do we lie low, or do we "surface" as taxpayers before the taxation authority (AFIP) each year to complete a tax declaration and test our protection under the tax treaty (which would not doubt be the technically correct thing to do). You will need to get a local tax adviser to tell you what the best approach is in your case. (The tax treaty might give you protection from tax on your pension income, but not from other imposts, such as assets taxes).