I am so sorry to hear this.
Depending on the situation, he may be moved to a hospice. He may be in hospital. If either of those you may want to be with him at the end if the timing can be judged and it is practical.
After the death before you can do anything you will need the Medical Certificate of Cause of Death. The attending doctor should sign this. You need to take it to the Registry Office covering the place he died, to register the death. There may be a delay in obtaining this and you may be unnerved by being told that the death must be registered within 5 days (not working days). Do not be alarmed, in practice it doesn't seem to be a problem. Inform the Registry Office if you can, even if you don't have the MCCD; this can often be done online. Google now, while you are fairly calm, to see if this is the case. You will still have to actually register his death in person. As PP said, several copies, say 6, of the Death Certificate (from the Registrar) can be really helpful and save the aggro of getting more. You may need to send them all off at once, but nowadays some places will accept a scanned copy.
You may prefer to find a Funeral Parlour in advance, ie now. Maybe ask around if anyone can recommend one. That sounds macabre, but a good supportive Funeral Director is a real blessing at a difficult time. You just phone them and they glide gently in and guide you.
It may not happen, but be prepared for your brain to stop functioning properly. It can be an awful shock and there seems to be so much to do and decisions to be made.
If people really want to help, ask them to do anything you need, eg phone the Funeral Parlour, fill in the Registry Office form (with your details), if applicable, drive you to the Registry Office, send emails to his contacts, or at least draw up a list, or post about his death on Facebook (as you want). Make sure there is a set of house and car keys easily accessible outside the house (maybe with a neighbour); it's very easy to lock yourself out while your mind is so busy.
What about the DCs? Are there arrangements in place for them to be looked after?
As PP say, the Tell Us Once service is good, and actually works. It's online and you fill it in step by step. Everything will be easier if you know where you can lay your hands on your DH's documentation: passport, driving licence, NHS number, National Insurance number. Also Bank Account numbers. Any accounts in his name only will be frozen, joint ones automatically become yours. But make sure you have access to money.