LLM72

Hello, I am a tax resident in the UK but not a UK citizen (I am from an EU country). I am currently working at a UK institution but I am also doing consultancy for a UN-system international organisation. This IO has sent me a letter stating that, as consultant, I am considered an employee and hence any income from this consultancy is tax exempt unless the UK has reserved its right to tax it. I checked the foundation charter of this IO and it says that the UK "retains its right to tax income earned by UK citizens". However, I am not a UK citizen and my EU country considers this income tax exempt. If I interpret this to the letter, then, am I correct that I am not supposed to pay tax on this income and hence not report it in my self-assessment? Many thanks in advance.