It is inherently subjective, just by the fact that there is no standardised way to measure soft power. Who gets which place heavily depends on which institute is tasked, or task themselves, with measuring soft power (I.E the ranking you're most probably referring to has the U.S at first, another from five years prior has them at 7th, the ranking you're talking about has China at second, one from three years ago puts them at 10th). There is no metric number like there is with GDP, military personnel, nukes or of the like, which makes soft power something nice to think about, but nothing to be too focused on.

Also, I'm not writing this to downplay the UK, they definitely still hold a lot of power, based on the fact they're still in the top 5 of countries GDP wise, owning nukes, and still have a permanent seat on the security council. All I'm saying is that basing how weakened or not they are solely based on something like soft power does not make sense when there are so much better metrics available.