Nancy I'm new to the forum - researching UK Carriers, who around the 1790s are based in Wolverhampton, not far from Sellack and also Sedgeley, home a a lot of 18th c. references. I can give you a direct line from me back to 1790 Wolverjampton, but not further yet.
Where did you find the Sellack entries? They are early and I've not seen them in IGI.
Have you got any evidence of a link between UK and US Carriers? Apart from dear old Thomas and Martha of Salem - for whom there seems to be no evidence of UK provenance yet.