What do you mean by "pull out"? If investors sell their shares then the company including all assets and employees still exist, it just has different owners. The actual obstacle to any separation from US control isn't financial but rather some level of dependence on US parts suppliers and licensed technology.
Rolls Royce is also responsible for building and maintaining the nuclear reactors used in Britain's missile launch submarines.
If things really got so bad that American investors began intentionally sabotaging the operations of foreign companies, it would probably be forcibly nationalised by the British Government.
in the context of this discussion, if all american investors pulled out of Rolls-Royce Holdings Plc because they were tasked with copying an american engine... could Rolls-Royce Holdings Plc make that engine?
This is what american isolationism means and will look like.