You'll quite possibly need the death certificate. Some providers will feel apprehensive about cutting off an old lady's fuel supply because of a phone call from a random. Power of Attorney comes to an end with the death of the person.
You can certainly do now -1) identify things you want to keep or might be mentioned in the will and set them aside 2) clear out rubbish 3) begin to sort (but see house clearance below) 4) look through papers for suppliers, the will, bank accounts and building society accounts, pre-paid funeral plan.
Your priority will be clearing the house and photographing meter readings, telling friends and family (delegate as much as possible - tell one family member and get them to tell the rest), and booking the undertakers - who will be a great source of support.
Some people have found it helpful to extract things of sentimental or high monetary value, then pay for a house clearing firm to do the rest.
Important to do as soon as possible - pension and other benefits so there won't be overpayments and masses to repay (google "tell us once"), anything else where money is being paid out.
Then without undue delay, the banks and building society, who will then freeze the accounts.
Don't actually pay out any money from the estate (except for funeral expenses) - this is "intermeddling" and could be problematical if you're not the Executor or if for any reason you want to back out of being the Executor. Even if you're happy to be the Executor, you can't pay anything out until you have established all the assets and obtained probate. I'm not a lawyer, but this is my understanding from having recently extricated myself from being an Executor in a particularly difficult case.