As I understand it, it goes something like: the list of symptoms which are required in UK for you to be able to get a free test is pretty much limited to cough or loss of smell or taste, but Delta has a different symptoms profile, so the more people get Delta the less the proportion of those who gets tested.
Here’s an interesting reasoning: https://covid.joinzoe.com/post/uk-cases-stop-rising
As I understand it, it goes something like: the list of symptoms which are required in UK for you to be able to get a free test is pretty much limited to cough or loss of smell or taste, but Delta has a different symptoms profile, so the more people get Delta the less the proportion of those who gets tested.
This wouldn’t explain the recent reduction in R because Delta has already been dominant for a while.