For me, Tell Us Once is very useful; it includes Blue Badge, passport office, driving, pensions, council tax etc. When my brother died a few years ago, because he died in a different district to where he lived, they couldn't offer Tell Us Once, and also it took longer to sort things out because I needed to get additional forms. It was a horrible time.
Re funerals; I found it straight forward to sort out funerals using independent, recommended funeral directors. What may help in anticipation of our own demise might be to write down what you would like for your final celebration, including music. For Mum's funeral last Friday, I was keen not to use the usual style of 'heavy' funeral director, but was lucky enough to find a local firm run by two ladies with a very light touch, but also a thorough and sensitive approach to making sure things went according to my wishes. No dark hearse, no Dickensian clothes, and I could call in to sit with Mum anytime during working hours. While researching undertakers this time round, I found out that a well known chain of funeral directors stores their clients in centralised 'hubs' rather than at the premises of the funeral director; I think the funeral director should tell you where your loved one will be staying.
Also, I found out this time round that different crematoria charge different prices! For my Dad last year, the cost of the crem, including service, was double that for my Mum using our local church then cremation at a different crem without a service. The lovely thing about Mum's funeral at our local church was that we were able to spend as long as we liked at the church and to make it ours for the day. I'm told that many funeral directors do not like to tell you about the simple cremation service.
Sorry I've written so much!