No, whilst you may choose the same person to be your Lasting Power of Attorney or Next of Kin, you will need to complete and register your Lasting Power of Attorney formally.
In law there is no such person as next of kin and so the only way to ensure that your chosen person(s) is involved in decisions about your care is to name someone of your choice in an Advance Care Plan. The person you chose does not need to be a relative. An attorney named in a Lasting Power of Attorney for health and care is the only person with a definite legal right to make decisions about your health and welfare if you have lost capacity.