Why capital-ship parity mattered more than cruiser advantage

Would Britain accept heavy-cruiser superiority if it meant slight inferiority in capital ships?

No. Britain could trade heavy-cruiser numbers for light-cruiser strength, but capital-ship parity was non-negotiable.

Capital ships were the politically decisive measure of naval strength in treaty negotiations. Britain might concede American superiority in heavy cruisers if it secured a compensating advantage in light cruisers. That trade reflected different worldwide cruiser requirements. It would not, however, accept a bargain that left the United States superior in capital ships.