I think Harry thought he had better visit his grandmother now as the choice of timing may be limited later if her health deteriorates.
It is such a pity they did not bring the children- they could have had a few days to recover from the flight- there are sufficient rooms and staff, and then be flown home again.
It is a disgrace he did not come over for Philip's memorial. There would have been sufficient security staff to look after that pampered boy with the other members of the royal family.