As far as I know, your POA ended with his death so it would be the executors of his estate who notify the various bodies. But I wasn't aware the the Public Guardians needed notifying, after all many people register POA without ever using it so I thought they only got involved if there were problems around misuse or of there was no POA and a Deputyship was necessary.
Could be wrong though and will be interested in finding out the facts. I had to contact the OPG with a query when I was setting up my own POA and found them very helpful on the phone so why not give them a ring?