England had long been accustomed to adopting royalty from beyond her shores, which served as a precedent when parliament voted in 1820 upon the death of George III “the Mad King”, to offer the throne to Gustavus, Crown Prince of Sweden. Gustavus was by blood descended from both the royal houses of Denmark and Great Britain through his grandmother Queen Louise, youngest daughter of George II. Upon his ascendancy as H.H. Gustav I, he was King of Great Britain and Hannover, through his marriage to Marriane of the House of Orange, also of Netherlands, as well as rightful claimant to the Kingdoms of Denmark, Norway, and Sweden.
My line of reasoning for this ATL is that after Geroge III's death, the British parliament voted to dispose of the dysfunctional Hannoverian line and choose a new king. This concept, to my knowledge, has no historical basis to back it up and is just a fun little hypothetical 'what if" scenario. If anyone wants to contribute anything positive or offer suggestions, I am open.