What a lovely gun. Nigel Brown's book places the manufacture right around the turn of the century.
More info is needed to ascertain a monetary value. WJ Jeffery&co. ispresently doing business in London and can be found on the internet. Contact them to see if there is any providence associated with the gun. Also,see if the gun is originally 2&3/4" chambers.
Photograph the barrel flats and post them so everyone can comment on the guns mechanical history. This will be important,and post the barrel measurements;bore and wall thickness,this is most important. If I left something out the next poster will fill in the blanks.
Justin