Britain entered the war before its shipbuilding buildup matured

How did British shipbuilding capacity in 1939 compare with 1929?

It was expanding and improved during the war, but Britain fought before its planned 1942 buildup was complete and lacked America's extra mobilization time.

British shipyards were reasonably capable in 1939, and their capacity continued to grow during the war. The expansion had been planned around a conflict beginning in about 1942, so hostilities arrived before the buildup was complete. The United States had roughly two more years of peace in which to expand production and achieved a much larger increase. With the same additional time, Britain would probably have entered the war less far behind American capacity.