If there are issues with your partner / ex partner, then yes certainly involve cafcass for the plane as you have done.
You may also need to consider a court order regarding custody and child arrangements, clearly the mother is going to cause problems.
A C100 child arrangements order may be the best way forward.
As she is on drugs, and making threats, clearly there is risk to you and the child.
You could therefore also consider a non molestation order against her, to stop her threats of violence and being near you and the child especially whilst on drugs.
The Lin here could help obtain such an order.
https://www.gov.uk/injunction-domestic-violence
As there are a number of issues here, cafcass, possibly child arrangements / custody, non molestation etc, you may consider instructing a family law firm to represent you. Legal aid may be available. The following link will allow you to look for a legal aid law firm.
Enter a local post code, select the area of law, ie family, crime etc and a list of local firms will be displayed, then all you do is simply call one.
https://find-legal-advice.justice.gov.uk
Also as she has made threats to harm you, that could also be reported to the police.