As a gun maker, it is not surprising to see John Sykes living in Greenwich Kent when Isaac was born in 1817. He may have been working at the near-by Royal Arsenal in Woolwich.
But since the children Mary Alice and Isaac both marry in the Tower Hamlets area in 1840 and 1841, it makes one think the family might have moved back to that area at some point.
Have you found Isaac Sykes in the 1841 census?