Thanks everyone for your views, and to Frank for the statistics, but I had meant something slightly different in my original post.
My experience is not of an occasional trickle of successful eBay searches from outside the area I mentioned, but nothing at all. Zilch. Zero. Over more than a decade.
So where's all the British glass in India, Pakistan, Bangladesh, Burma, Nepal, Malaya, Borneo, Singapore, Hong Kong, Ghana, South Africa, Malawi, Mozambique, West Indies, Jamaica, Zimbabwe, and Aden?
... and apologies for that rather garbled list, derived from a 1930s Pears map of the world, and my memories of stamp collecting.