Never rule out the tangata whenua, particularly on their own ground. The New Zealand Wars required a massive commitment of troops in OTL- 20, 000 at one point, which is absolutely ridiculous given the size of Aotearoa and the forces they were fighting. Even then, Britain required a complex series of alliances with other iwi to secure victory.
I don't think Britain will lose, unfortunately. But I can easily picture 'veteran' troops not taking their enemy seriously and blundering into another Gate Pa.
This is a widespread belief, but not grounded in fact; over twenty years ago the New Zealand historian Philipa Mein Smith demonstrated that the Australians were prepared to make any constitutional alterations necessary to let New Zealand treat the Maori differently, as everyone agreed that they were of a 'higher' race than indigenous Australians. While New Zealand was concerned about this to some extent, if anything New Zealanders at the time were more worried that Australia wouldn't remain racially pure in the face of Asian migration.