The appointment of your chosen attorney will not commence until your LPA has been registered with the office of the public guardian.  You can apply to register your LPA at any time whilst you have capacity, but if you lose capacity only your attorneys can register your LPA.  It is strongly advisable to register your LPA as soon as it has been completed and fully signed.
If you have chosen to allow your financial affairs attorney to act before you lose capacity, they can act as soon as your LPA has been registered but this can only be with your consent.  Once you lose capacity, your attorney can act without your consent (as you will not able to give or withhold consent).   Your health and care attorneys can only act after you have lost capacity to make health and care decisions for yourself.