You say the children are subject to involvement of Children and Family Services. Is that on a voluntary basis, an agreement with the children's parents, or is there an Interim Care Order. Have you instructed a child care lawyer? That's essential.
It's in the children's interests, and yours for there to be an ICO. Sometimes l.a.'s avoid care proceedings to save money and worse, may tell parents "agree to us putting the children with your mum / in foster care, or we'll go to Court". Much better for there to be a Court. Your grandchildren are young but need decisions made in their timescale.
If you're as honest with the social workers as you have been here about resentment and not wanting to commit to them for ever, the decision may be taken for you and the children place for adoption. I'm not being critical of you, you're in a rotten situation.
To think that London, or anywhere else for that matter, does not belong to any one demographic