So higher rates and better tax collection led to more revenues to be collected. The Laffer thesis more or less says that low rates lead to more tax revenues since low rates produce more growth and more growth increases the base to be taxed, while higher taxes drive people out of the market and end up lowering revenues. Leaving aside the evidence (or lack thereof) for the accuracy of this thesis in practice, I take it that MR wants us to understand that revenues did not rise as a result of increased growth producing a bigger base and therefore more revenues at the same low rate as before, but rather England raised its tax rates and collected more taxes as a result. Laffer doesn't seem to have played any role so I'm not sure what the point is of bringing him into this at all. All in all I am finding MR more than usually confusing the point with this note, but I appreciate that they alerted me to the book. Better just to read that, I think.