According to Eley's US Website, the Ventus pellets are being made by Eley themselves in Birmingham. In which case it would follow that the Tenex pellets are also their own.
Of course the US tend to play faster and looser with advertising standards. They don't explicitely say Birmingham on the UK website, though they do use language to suggest they're their own pellets:
Using a word like "make" would be interesting to the ASA if they didn't actually manufacture the product.At ELEY we make all our pellets to extremely tight weight tolerances and can claim over 30% superior weight variance within a batch.
I did have it in my head from somewhere else that they are indeed making their own pellets, but I can't recall where or from whom I heard it.