I'm a little confused. You say that you have got your granddaughter again - how did that happen? I would assume that your daughter has asked you to keep her, in which case I don't understand why you're being threatened about what they will do "when they pass the drugs tests". You're not stopping them from seeing their daughter - it is they who are not making the effort.
And who will be administering the drugs tests? They are eye-wateringly expensive, so aren't done lightly.
I think you need to get some advice for yourself and your granddaughter, as it sounds as though she would not be safe if she was just returned to either, or both, parents.
Unfortunately, the referral from the police will have to go through the police system before being made to social services, and will then again have to go through social services' system before any calls are made to you. And actually, because you don't have parental responsibility for your granddaughter, they may not call you - data protection etc etc etc . They may say they need consent from a parent with PR to talk to you.
However, if you were to call them, then they could listen to all your concerns and worries although they may not be able to discuss the child with you - despite you having her in your care - and could then make further enquiries.