Vauxhall styling was heavily influenced by America, perhaps more so than Opel. They were regarded as 'a bit brash' by the notoriously conservative British buyers, who loved buying the stodgy British offerings at that time, e.g. most BMC cars.
When GM wanted something a bit more up-market in the UK they started to import Opels and sell them as 'premium' cars (at appropriately higher prices); most people in the UK had no idea they were so closely related to Vauxhall of course, but just viewed them as premium German cars..
They seemed to sell quite well and I bought a few Asconas and a City Kadett for my company drivers to use, but the experiment was not specially long-lived and once Vauxhall reckoned their cars were premium in their own right the importation of Opels stopped.
The public never agreed with Vauxhall, however...!