So sorry for your loss
i had trouble with my husbands bank -they contiued to pay direct debits out of his account despite being notified and sent copies of the certificate on three occasions - My stepson ended up throwing a wobbly, taking his ashes into the branch along with yet another certificate, before they closed the account, and then they tried to get me to pay the deficit - i was told as it wasnt a joint account i was not responsible, and as there was no estate to pay it they didnt get their money - the car insurance company trippled my premium even though it was my insurance in my name for the same vehicle - i am no longer with them and wouldnt even consider that particular bank under any circumstances.
Sadly companies can be like this in part i think because they just do what the computer says and the system dictates - the letters they send out they dont even read - neverless it adds further heart break for those of us at the other end of them at a time we are at our most vulnerable.
Assuming you live in the same house and this has happened because you applied to transfer the bill into your name it involves closing one account to open another -just as they would if he had moved house and a new person was moving in - therefore it really isnt a debt as such - it is for a bill he would have paid had he been able too, so try not to think of it terms of debt even if it is worded that way. I dont know how they can pass an estimated claim on to a debt collection company either, surely it has to be a specified amount and the meter reading disclosed and actual bill calculated at that time when you asked to change the account too your name so you are right this seems very strange.
i think the legal implication is that they need to make a claim on the estate they cant make one on you personally.
If you have a solicitor or some-one else dealing with probate then ask them, and pass the letters on.
If you are trying to sort it out yourself then try citizens advice first as i am not a solicitor so could be wrong, but as far as i am aware we are not personally repsonsible for "debts" incurred by a spouse it must come from their estate. dont be afraid to contact the "debt collection company" and inform them that you have no power to access your husbands bank account until probate has been granted - I know its really difficult to think of something your husband took pride in being on top of as being an issue now - but that company, despite their main function might actually be more helpful and supportive than the electric company, it could be they genuinely have a system by which they pass this on in order to and deal with beravement and estates seperately from the way other causes of non payment situations are dealt with.Also point out to them this oddity of an "estimated" amount.
It is awful to have to deal with this stuff my heart goes out too you i found it helped in a peverse kind od way i guess to think of it as still fighting on behalf of my husband.