All the warships that were in the Admiralty's 10-Year Programme of 1924 that weren't built IOTL. In particular the 3 aircraft carriers that weren't built.
IOTL the plan was to build four 17,000 ton aircraft carriers to replace Argus, Eagle, Hermes and Vindictive. The following might not be 100% accurate, but from memory they were to be ordered in the 1924-25, 1928-29, 1931-32 and 1934-35 Navy Estimates; laid down in 1925, 1929, 1932 and 1935; and be completed in 1928, 1932, 1935 and 1938 respectively.
Therefore, the Royal Navy would have had seven aircraft carriers at the end of 1938 consisting of the four 17,000 ton ships and the "Three Follies."