The Vauxhall are a different sort of police car. Astras and insignia etc are Incident Response Vehicles, so the ones that turn up to burglaries, RTCs etc.

The N57 powered BMWs are a mix of mostly Roads Policing Units, Armed Response Vehicles and Area Cars (in places that actually have this class). These classes all have performance requirements beyond Vauxhall range, I believe they are mostly being replaced with assorted Volvos and Touregs in some areas.

That being said, the headline is misleading, as what is actually shutting down is Park Lane BMWs service vehicle division. BMW is likely to continue to supply vehicles, just not through that division. I wouldn't expect BMW police cars to dissappear any time soon.