it's PSA in France who build the 4-cyl diesels as part of the Ford/PSA engine alliance.

Well, they cast the block, head and bolt the parts together.

But all the expensive goodies (turbo and CR fuel system, and possibly the pistons will be bought in.

Then you have to consider the-arguably more important-intellectual provenance of the car. ie where it was designed. These are the sorts of jobs we want to be encouraging rather than low grade production line work

To be honest, it's very difficult to determine "how British" a complex product is.

The Astra is mostly built from parts made abroad (Spain, Portugal, Germany), that is why the Vaxhaul plant was considered at threat

Ellesmere Port is the lead plant for Astra and all it's world-wide derivatives.

GM Europe has excess capacity due to the major downturn in European car sales - the reason Ellesmere Port is being considered for closure, not for the first time, is that French and German governments and trades unions make it much harder to close plants there - having closed the Belgian plant because it's easiest/cheapest then Ellesmere Port is the next easiest/cheapest to close.

Opel have plants in Spain and Poland also although I did not know they had plants in France.

They don't - I was thinking of PSA where the same thing happened.