In the past ten years or so Vauxhall have upped their game, and can hardly be regarded as truly downmarket anymore, they now build some very desirable models. However they still have some way to go to be regarded as top notch as the Germans.
Chevy are a cheap brand though. As for the two brands competing against each other, surely it wouldn't be a problem, Ford and Volvo competed against each other not so long ago for example.
The BTCC frankly needs a few manufacturers to come back, Vauxhall have been the stalwart for so many years now, and its a shame they pulled out, it would be very good for the championship if they were to return, we might even see a brand spanking new Insignia on the grid soon.