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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.
Indeed, it would cost more to make the Opel make known in UK than what it costs to make different grille and badges for the cars
Lots of Australian Holdens are just rebadged Opels as well, although they also have original models.
There are also rebadged Isuzu/Suzuki, but that's still GM.