What carriers would Britain build next?

Would Britain crash-build more Ark Royal-class carriers for the war against Japan?

No: it would finish the Illustrious class, then build larger fleet carriers and more Unicorn-like support ships as experience accumulated.

Britain would probably continue the Illustrious class instead of starting a crash program of additional Ark Royal-class ships. Combat experience would push later designs toward much larger air groups, producing ships closer in scale to Audacious or Midway. Long Pacific supply lines would also create demand for additional Unicorn-like maintenance and support carriers. The likely response is therefore a mixed carrier system rather than repetition of one prewar design.