There are 2 entries for Stephen Portch in the 1851 census. One is living with the family of George and Sarah Portch in Broadway, Frome. The other is with Hester Portch in Nunney.
In the 1881 census, there are also 2 entries for Stephen Portch. There is a Stephen Hunt Portch, born Nunney, age 46, living with wife Elizabeth, age 50, born Bristol, plus 5 children, in Goswell Road, Finsbury. Then there is a Stephen Portch, age 47, born Frome, Waggoner, living with wife Elizabeth, age 52, born Kingston (?) Herefordshire, there are no children, address is 38 Grove Street, Wolverhampton.
It seems a very confusing picture as we have 2 men by the name of Stephen Portch, both with wives by the name of Elizabeth, living in different places in 1881. The Stephen Portch in Wolverhampton, looks to be the son of George and Sarah Portch.
The London Electoral Registers show Stphen Hunt Portch living in 8 Goswell Road, Finsbury in 1883.
I think you can be certain that your Stephen Hunt Portch was living in London.
Jo.