UG might be from the United Glassworks
Just found this post as I'm trying to research an empty bottle of Walker's Kilmarnock Whisky (marked Dummy Coloured Water) that I dug from my garden.
I'm thinking that the 'UG' on your bottle might indicate that it was produced by one of the United Glass Bottle Works. There was one in Shettleston,Glasgow;Portobello,Edinburgh and-closer to where I live in Dunblane- in Alloa and Bridge of Allan. And there may have been more? The works in Glasgow (closest to Walker's in Kilmarnock) closed down in 1982.
Hope this info is of some use to you?