OK,the gun , to me, does not look like a gun made off of the Continent. Remember in the early pinfire and centerfire days, there were English marked guns which originated on the Continent. That warped sideplate seems like a giveaway.
Is it my memory failing or did I read once in Shorting Gun or Shooting Gazette that English required stamping "not English Made or not English Make" to protect domestic manufacturers from Continental competition. I think that was done in 1920s to protect their market from flood of inexpensive German imports?
I'm not saying what he has there isn't keeper grade Birmingham-made gun, but to experienced eye there isn't a whole lot of gun there, therefore, it was likely stamped "England" to help distinguish that from mainly Belgian competition sold in England at that time.
It does kind of remind me what Winchester imported into USA as their first SxS guns. They were hammer guns from Brimingham trade from likes of Redmond, Tolley,.....