Your power of attorney means you will make all decisions whilst he is alive (assuming you have both health & welfare and property & financial POA).
If he died, you, as his executor, would be responsible for organising his funeral and dealing with his estate.
Regarding organ donation, if he hasn't recorded a decision, the starting position is that the donation should go ahead. The family will be asked to support his decision before any organ donation goes ahead. That would normally include you, especially if your husband dies in hospital and names you as his next of kin when he is admitted.
Are there any other decisions you are worried about?
UK law does not have any clear rules around who is your next of kin. However, as you are not married or in a civil partnership, if he were to die intestate you would not be able to administer his estate, nor would you be entitled to inherit. As you have made wills, that is irrelevant.
Being his long-term partner does not give you any rights. If you really want to be sure that everyone will accept that you are his next of kin, the only way to achieve it is to marry him or enter a civil partnership with him.