How to help someone post brain injury with substance addiction now not answering calls.

[BrightRedSheep]

Adult had brain injury and is spiralling into drug and alcohol addiction (and debt!)
They are not answering phone or responding to texts - only reach out when money is needed.
Many concerned friends and family members are desperate to help.
Does anyone know of any way support can be found when someone is unwilling to stop and allow help? (inUK)
The brain injury has resulted in no impulse control.
Any advice most gratefully received.

Headway is the charity organisation for people affected by brain injury and their families and carers and they have a helpline, so I guess I'd start there. Good luck @BrightRedSheep , sounds very distressing, I hope there's a way through for you and the injured person.
https://www.headway.org.uk/supporting-you/

Is the brain injury thought to have reduced/negated their capacity? Do they need practical support now in a way they didn't before?

You could request the police do a welfare check if you cannot reach them.

Does anyone know of any way support can be found when someone is unwilling to stop and allow help? (inUK). The brain injury has resulted in no impulse control.

Just to highlight the obvious point that if their brain injury has caused the problem, then they do not have it in them to stop or to permit help. Willingness is not in their control. They can’t do it. So the PP who asks about capacity is spot on. What support do you have in mind?