While Britain was in the Thirty Years' Peace the PU was fine, but once war broke out and they realized what a liability Hannover was (for instance, they couldconquer the rest of Europe's overseas empires, and all that someone, say France, had to do, was march into Germany, hold Hannover hostage and Britain was forced to the negotiating table right quick), but that's taking George I-II's preference for Hannover over Britain into account. If Fred appoints Cumberland as regent for Hannover and doesn't really further bother with it, Britain might take a different view of Hannover.
Interesting. They really didn't think that through when they created the Act of Succession.