hmmm...interesting. To make this work I think you have to consider the way that the soldiers look at their jobs. The KGL so vaunted during the Napoleonic Wars fought in British service because they were doing their duty with the anti-French coalition to secure freedom for Hanover. They were fighting for King and country, and really were an independent brigade under British leadership. The Gurkhas are an established British regiment because they fought for the British Empire for centuries, and because their opportunities of fighting with a world-beating and highly-advanced army and being recognised as an important part of it, are limited in Nepal. So I think that to do this you need one of two scenarios: either Hanover remains in union with Britain, and their armies become incorporated (which probably suggests a closer level of union between the two states) or you need Germany, or at least Hanover, to be militarily insignificant, making the opportunity to join the British army become a real enticement - this means spiriting away German unification, and probably making Hanover have a really hard time and lose a lot of strength and prestige in wars over the last 150 years.