Hello, I am Keith, one of the experts on Just Answer, and happy to help you with your question.
Firstly, when you left the UK did you send a Form P85 to your tax office. If you did not then you should do so now, fortunately there is no time limit for its submission, the form is available on the net and can be filed on line. HMRC will then classify you as non-resident.
I am in total confusion as to HMRC's action referring to Inheritance Tax (IHT). IHT is only levied on death, but it can follow you after you have left the UK if you are domiciled in the UK. From the tenor of the question you are not domiciled in the UK and never have been; more likely Ireland or posibly Australia, but not the UK. In the UK it takes about 16 years to establish domicile and with only 7 years of residence you would never be so domiciled. There can be IHT complications with EBT schemes, but without more detail I cannot assist at this stage.
You do not need to claim non domiciled status, it is for HMRC to show that you are UK domiciled, which clearly you are not. HMRC are very hot on changhes in domicile, but they cannot have it both ways.
You may need trusted, UK professional assistance in this matter to fight your corner..