There was something posted here last year about sales from a range of independent stores not getting counted in future years, so I can quite believe it.
How you would correctly estimate sales under the old system without carrying out the old measurement, I'm not sure. Some of the people left out in the new measurement would have an interest in talking the alternative number up, as well.
Given the sales of a few big artists in the first half of 2024 (you all know who I'm referring to here, I'm sure) I would not be surprised if, should someone calculate 2023 sales by the new method, the increase for 2024 turned out to be a lot more than 6%. I was trying to find numbers before responding.
I saw some articles suggesting that UK sales of both CDs and LPs may be heavily undercounted, and that the UK charts have been unrepresentative for several years, as different companies and artists have to pay the company compiling the "official" charts to get different channels of sales and streams counted. I've not found anything I'd be prepared to post, but it looks like the sales figures I quoted to show that the UK was behind the US in terms of the sales revival, just might be fantasy.