What is the difference between a contract that is governed subject to the exclusive jurisdiction of the courts of England, and one that is subject to their non-exclusive jurisdiction?
When jurisdiction is exclusive, ONLY the courts of England have authority to adjudicate relevant disputes.
When jursidiction is non-exclusive, then the courts of England are granted such authority, but other courts may also be able to adjudicate, depending upon the (somewhat complex) rules of private international law.
In some cases (e.g. consumer protection cases) a jurisdiction clause will be overridden by the courts.