For those who prefer to think in pictures, this chart from the OBR's 2018 report "Why have onshore CT receipts performed so well since 2013-2014?" is a very neat illustration of the balance of measures that have more or less offset each previous cut of the corporation tax rate so that the effective tax rate has remained broadly flat.
This is why the raise in the corporation tax rate announced on 3 March 2021 is not simply rolling the clock back to c. April 2012, when the rate dropped from 26% to 24%, but a very substantial tax hike (>30%). The above-the-line measures on the chart below, together with those introduced after this chart was produced, will remain in place.
You might like to think of the impact as the equivalent of removing almost everything from one side of a very balanced set of scales. That is not to say it is a good thing or a bad thing per se. But we do need to recognise that it is a new thing, not a return to a previous position or the simple reversal of previous changes.
Although I suppose in one respect it is demonstrably a good thing. A 25% rate makes calculations easy enough that even a lawyer can do them...
But we do need to recognise that it is a new thing, not a return to a previous position or the simple reversal of previous changes.