Let the watch idea go. If the girl is your son’s child then she is next of kin and the watch really belongs to her and effectively is still passing down the family. It is a very traumatic time and thoughts are all over the place. Having lost a son myself I know.
Be pleased that your granddaughter has some small trinket of her father, even if his partner is wearing it for comfort at the moment.
My sincere condolences.
To think that London, or anywhere else for that matter, does not belong to any one demographic