Truly there isn't much workmanship: a press mould and a lady (almost certainly) on piece work with sticky transfer strips and some trays of powdered glass. Dexterity and speed almost certainly but little workmanship
In my book I mention that the production of a range of cast-iron moulds was a very expensive business, meaning the manufacturer would not have taken the decision lightly to produce just a small run. Each mould and plunger would cost hundreds of pounds (sometimes thousands for the more complex 3-part moulds) in the 1950s and when we talk of mass-production, this means a conveyor belt having several of these moulds - sometimes dozens for the smaller items. Given that the value of a pound sterling (£) in the mid-1950s is worth about 40 times that today, the investment was massive. Adam, though, is better qualified to talk of Sowerby's output, having worked there from 1949-56 - just the time when these dishes were produced.
The implication therefore is, as stated by Christine, that they are not rare and, as mass-produced items, there were thousands and thousands produced, not just ones and twos. Because they were of little value, yes, they were often consigned to the bin or the local junk shop when granny got down to two or three items. Naturally, some designs were more successful than others, but invariably the best term you could apply to mass-produced, press-moulded glassware is "scarce".