Why Britain would build heavy cruisers before the 1930 treaty

How could the Royal Navy secure a very large interwar cruiser force under treaty limits?

Build the heavy cruisers before the 1930 treaty, then use the established tonnage to argue for a matching light-cruiser allowance.

Before 1930 the navy could build additional County-type heavy cruisers without the later category limit. Entering negotiations with roughly 320,000 to 360,000 tons already in service would create a much stronger bargaining baseline. Britain could then argue for a comparable allowance of light cruisers rather than trying to create the heavy force after the rules tightened. The final mix would still depend on design experience and the fleet's actual requirements, not treaty arithmetic alone.