Pontiacs are dressed up quite a lot (on the outside, at least), compared to other models on the same platform. I get the impression that to make a Vauxhall out of an Opel they just move the steering wheel over and swap the badge.
It probably doesn't cost them a lot to maintain the two different brands, since they would have to make different advertising for the UK market (in English) anyway.

antp, I don't think it'll take that long for Saturn to move over, the only thing they sell now that isn't an Opel is the Outlook.