NoForn
There might be a difference between the manufacturer not releasing all code to a customer as it is commercially highly sensitive, and a government slapping a No Foreigner restriction on a deal.
The Forbes article doesn't state which has occurred and flits between quoting the company rep and using the phrase 'The United States'.
If Drayson had secured access prior to signing the contract and now the deal is changed, the MoD should consider whether it is in the country's interests to bang out.
Perhaps BAe Systems and Rolls Royce need to with hold some of their 'bits' back and start the first North Atlantic trade war between the EUSSR and the US