Hi Mat - didn't want to make my reply too lengthy - which would have been easy in view of the very interesting history surrounding this particular coloured glass. But............
Blue glass, made in the U.K., was achieved by using smalt smuggled from Saxony in the early part of the C18, although apparently this export was banned from Saxony around 1830 because of the high value of the mineral.
Blue had been used on the Continent long before being used in the U.K.
At some point in the 1760's, the Prussians - who were occupying Saxony at that time - sold the Royal Saxon Smalt Works entire stock of smalt to an Englishman, and the import to the U.K. through the west country port of Bristol - plus the fact that it is first recorded as having been made in that city - are reasons for it being so named, although subsequently it was made in a variety of places.
The aforementioned Jacobs was clever enough to give the King a gift of some gilded Bristol-blue glass, and in so doing styled himself 'Glass Manufacturer to His Majesty', and became the first guy to benefit from the value of word association, since he had combined Royalty with the city of Bristol, and never looked back.
The other name associated with Bristol-blue, was the glass decorator and gilder, Michael Edkins from the same city.
The original U.K. stock of smalt appears to have been depleted around 1804, and the Napoleonic wars blocked further imports so home grown supplies were used until imports were again available c. 1815......... and so it ticks on - but a fascinating history of a great colour, and well worth reading.
Worth adding that ...... in general for British made decanters (and some tumbler type drinking glasses), produced during the first quarter of the C19 ......... the most commonly found base should show a large diameter shallow depression (where the pontil scar would have been). The outer rim should show a dull mat surface created by much wear.
I suspect this info is all on Wiki, but I've taken the liberty of making a rather brutal precis of some of the info in Andy McConnell's wonderful book 'The Decanter' - I know it's an expensive book, but worth every shekel, if you can source a copy.
Apologies for being long-winded, again
