Nice find Keith! The thing that surprised me about these jugs (I have a Calypto one) is how heavy they are! They are big too...
Remember there are two versions: one with pontil (heavy); one without (much lighter and only seen in Silhouette so far). This dates the lighter version to 1967 or later - so was this the first production run, given that Sovereign Rose uses the heavier base and is dated to 1972? To the best of my knowledge, I now have every single example of the pattern, so only used those scarcer items for the book - so add Anemone, Swirl and Calypto.
I'm sure Chance would have had the capability and facilities to make these jugs, but it was sheer expediency to buy them in. I assume this, as it would have meant allocating staff to make these items, which were not purchased in vast numbers. In fact, it is entirely possible that Chance bought just one or two shipments. I'll check with Tony*, but Chance workers were quite adept at turning their hand to making anything in glass.
Christine is right though: shipping these Portuguese jugs in (yet to be named) was the cheaper option, and I understand that none of the British companies could compete.
*
Who'll probably remember something else that can go in the book 