Prologue
June, 1810:
After an offer of being elected the next King of Sweden, the very popular Jean-Baptiste Bernadotte had a choice to make. He had relayed the offer to his Emperor, and his Emperor had showed disapproval [1] - urging Bernadotte to not leave his new station as Governor of Rome. In the end, his loyalty to the Emperor rose above all, and he declined the position.
Frederick von Dahl [2], who was a lesser member of the Riksdag of the Estates and a critic of Napoleon and the French Empire, persuades the Riksdag that a pro-British King would be best. He argues that a soldier is needed as King, similar to Bernadotte, but not a soldier loyal to Napoleon, a soldier who is
against him. A tactician who could turn the Swedish luck upside down, and regain their status as a Great Power, which he states was lost after the Russians stole Finland from them. He begins secret communications with Spencer Perceval, the Prime Minister of Britain. Perceval relays an offer to Sir Arthur Wellesley [3], a known war hero. It takes several weeks to get the offer to Arthur, who accepted almost immediately.
July 1810:
Charles XIII of Sweden comes down with a bout of sickness and dies within the week [4]. All of Sweden mourns his lost, even if only halfheartedly. The Riksdag is forced to hurry things along and pick a candidate; Von Dahl continues campaigning for Wellesley, and it seemed as if they would choose him. The King of Denmark had declined taking the crown, as did Bernadotte - however several Germanic princes, such as the Duke of Oldenburg, had accepted to be candidates, as well as Gustav von Holstein-Gottorp, the son of the former King.
August 1810:
Despite the vast amount of others who could, in theory, become King of Sweden, in August 20th, 1810, Sir Wellesley is elected by the Riksdag of the Estates as Crown Prince of Sweden and Generalissimo of the Swedish Armed Forces. When news hits the rest of the world, several nations object. Bonaparte was furious that Sweden had a Brit on the throne. Frederick William III, King of Prussia and Alexander I, Tsar of Russia both were against it, however they keep their grumbling quiet for the moment; Frederick William could not, despite his wishes, muster up the strength to declare a war, while Alexander (who had recently began negotiating secret treaties with the Brits) was preparing to end his alliance with the French, and another war against Stockholm and London would be counterproductive. Instead, he publicly backs Wellesley, adding more tension between Russia and France. Alexander sends a courier to Arthur with a message, hoping to pressure him into joining into the secret treaties. Wellesley, soon to be in control of Sweden, accepts - and that treaty would be put to use very soon, when Napoleon turns his sight towards Moscow.
September 1810:
Arthur Wellesley enters Stockholm in late September, and is crowned King Arthur I [5] of Sweden. The Swedish Army is overjoyed at having an officer of such renown as King after the failures of the past ones. However, even as the new King is celebrated, enemies to the south prepare for war.
[1] POD, of course, is that Bernadotte rejects the offer, because Napoleon disapproves. IOTL he did not approve, but didn't disapprove either.
[2] Frederick von Dahl is a made up character - he serves as a placeholder, as I can't find the names of any actual pro-British members of the Riksdag.
[3] Arthur Wellesley is sort of an unlikely choice to be King of Sweden, but he was a soldier, which pleased the Swedish Army. It will tie Stockholm heavily to London, indeed.
[4] The King dies early.
[5] Since the King died early, there was no adoption, and no name change to Charles - this breaks the precedent and tradition.