Why Britain could build more than two super-battleships

Would 70,000- to 80,000-ton battleships limit the Royal Navy to only two ships?

No; continued battleship construction would bring matching investment in yards and industry, allowing a larger class if policy and funding demanded it.

Britain possessed several yards that could already take very large ships or could be expanded to do so. In a treatyless world where battleships kept growing, infrastructure and armor production would grow with the designs rather than remaining frozen at historical capacity. British ordering practice also favored one experimental ship, two test examples, or larger frontline classes. An arbitrary pair would not become the permanent battle fleet.