I always think that the Altaglass freeform items are much 'whackier' than the items normally found in the UK. I also think that because of the shape and size of most of these freeform items, they would have been a nightmare to ship from abroad (certainly not in a suitcase!) and the reason why being British-made does make some sense. But, conversely, I've never seen mention of these things in Pottery Gazettes or Pottery & Glass trade journals, and I've scoured virtually every issue (also have a few copies) from the late 1930s right up to 1981.
Of course, it is possible they were the output of a smaller concern, who just bashed them out for a few years and were not concerned about offering them through the trade. But why the Davidson label? I think until Chris Stewart reads this topic we are thrashing about in the dark a little.