Here is an interesting what if scenario.
Before Queen Victoria ascends to the throne the British get the Salic law in Hanover annulled. The personal union remains in effect through Victoria's reign. The Austro Prussian war comes, Queen Victoria declares Hanover's neutrality and sends troops to protect said neutrality.
After the German Confederation is abolished the British Parliament votes to annex Hanover into the "United Kingdom of Great Britain, Ireland, and Hanover".