I am no expert on the value of UK bottles and am coming up short finding any more info on this particular mold - Stephens was in business for many years - see this timeline of the company which provides about as much detail as one could expect to find on any 19th century business.
I did find several examples of the same square neck mold but without the vertical embossing down the body of the bottle. I would assume that has some contribution to value.
