ACT: tax credit & FA93: reason for the changes
The reason for the changes introduced by FA93 was to raise revenue for the Exchequer. AUK resident taxpayer who was liable at the higher rate of 40% suffered an additional liability of 20% (40% - 20%) rather than 15% (40% - 25%) on the dividend income. Furthermore, a body that was exempt from tax (such as a pension fund) received payment of any tax credit at 20% rather than 25%.
The intermediate ACT rate of 9/31 for distributions made in 1993-94 was introduced because an immediate reduction to 1/4 would have had a significant Exchequer cost.