Hello, Doghouse, and welcome to TP. I'm sorry to hear about your mother and her diagnosis and your upset. However, I am glad you found your way here. I've only been around for a year but have gotten so much help, advice, information, and support here from TP. I hope it will be a good resource for you as well.
As I'm in the States, I'm not the best one to advise you on procedures in the UK but others who are more local to you can certainly do so. As lin said, we don't want to overwhelm you here at the beginning, but there are some things you will want to put in place sooner rather than later, now that you know what you are dealing with, and sorting the powers of attorney should go on your to-do list.
But of course right now I can completely understand you feeling a mix of relief and uncertainty and just plain feeling upset and overwhelmed. I am sorry your mother was upset by the news and hope her anxiety and distress from this get better. The good news about getting a diagnosis, as awful as it is, is that you know what is going on and you can get some things in place to help your mother and you and your family. (But then, I'm a person who would always rather know what is going on, no matter how terrible and I appreciate not everyone is like this.)
Don't worry about feeling upset or like you are complaining or moaning here. TP is a very safe place to talk about whatever you need to discuss, any time, day or night. For many of us, it's a huge relief just to know we are not alone. We do know that feeling of isolation, but you're not alone here.
Wishing you all the best.