Hello my name is ***** ***** I am delighted to answer your questions today. I have 35 years experience as an immigration lawyer.
No, you are not in the category of immediate relative that can sponsor your grandchild.
You must be a parent, spouse, sibling, or adult US citizen child (sponsoring a parent).
Your son can petition his daughter but it will take about 2 years for a visa number to become available.
He will have to establish a domicile in the US at that time when her priority date is current and a visa is available.
You would want to read this link which explains what domicile is under the immigration laws.
Scroll down to "Sponsorship Requirements" and under that is "DOMICILE".
Her father as a permanent resident has to be cautious not to loose his permanent resident status by living abroad. Brief "visits" to the US is not maintaining a residence there and may be in danger of the Customs Officer finding him to have abandoned his status. (Just a side note of warning since you said he was "living" in London.
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