It could be more than the weak pound that keeps them open. We build stuff better and cheaper than the French, also (unfortunately) its easier to hire and fire UK workers and the use of 'agency' workers is not the norm in France.
The problem is French politics....the Peugeot family and the French government own a massive chunk of PSA and are more likely to think with their hearts (self preservation) than their heads (solid business sense)
Ellesmere Port, which builds Astra cars for both GM’s Vauxhall and Opel brands, is seen as especially vulnerable to any cutbacks following a deal, since it exports around 80 per cent of its production to mainland Europe but imports around three-quarters of its components. Such an arrangement could be highly vulnerable to any imposition of tariffs following the UK’s departure from the EU.
What I found interesting was:_ Even prior to the sale to PSA, Opel's European boss Karl-Thomas Neumann is believed to have created a plan to reinvent Opel and Vauxhall as an all-electric car brand by 2030, with models based around the modular electric car platform used for the Chevrolet Bolt.