The metaphor isn't that bad. The interpretation is bad. The claim that measuring inflation (1) between last year's prices in pounds and this year's prices in pounds, or (2) between last year's prices in pounds and this year's prices in yen, differ in any significant way, is a bit of nonsense that should discredit the author. The entire concept of inflation is that the value of the monetary unit differs from year to year, and you want to find out what the difference is.

And similarly, the notion that British grades from this year can't be compared to British grades from two years ago is also nonsense. They are fundamentally similar phenomena and they can be compared. For political reasons, much less data was collected this year. That doesn't mean this year's data is different in kind. It means this year's data is low-quality.