Here are just a few of the US companies advertising in the British Pottery Gazette ads (circa 1905 - 1930):
1. Jefferson Glass of WVa were advertising their Krys-tol (through Pratt's agency)
2. Imperial were advertising Carnival through Markt.
3. United States Glass Co., were advertising (I think they had their own agency).
4. The British American Glass Co. were advertising (own showrooms in Hatton gardens).
5. The National Glass Co., were advertising the Chippendale range and Fenton Carnival.
I hope this helps to add to the bigger picture.