Make Darlan viceroy, just because Britain's a naval power, then pack the administration with Welsh and Scots who will create resentment in the English, leading to the formation of crippling inter-ethnic feuds which will tear the island apart. Then in about twenty years release the UK as an independent country in which the English are subservient to the Scots. This will hopefully lead to a series of crippling civil wars and eventually the partition and Balkanization of Britain, forever removing the Britons as a viable world power.
