Why not? UK correctly recognises that the economic centre is shifting to Asia and it obviously wants to enter a high-quality trade agreement with them (as China also wants as well)
Now I am not expecting many benefits for the UK but it will still give them a seat on the table.
And most importantly, it will manage to stonewall any China attempts to influence the CPTPP.
With Australia, Japan, UK and Vietnam(behind the scenes) working together they will manage to counter a bit of China's power.
This is good news for the UK and even better for the US
I am pretty sure that the US is satisfied on how their lieutenant, UK, has been working so far. Great job!
UK actually started its own version of Pivot to Asia much earlier - back in 2015. It took the lead among western countries and joined AIIB, against US wishes. It embraced with what it believed its golden age of UK-China relationship too back then. I suppose Brexit and the US-China strategic competition have killed them. Now, having turned itself against EU, its largest trading partner and China, UK has found its options become very limited. Its only choice now is to hook up with the US bandwagon. Now it does a Pivot to Asia version 2: apply to join the club originally design to contain China in trade, the (CP)TPP, and send an armada to Asia-Pacific, not so much to flex muscle against China but to show loyalty to its American boss.
I doubt all these have much of any impact on other nations especially China, or much benefits to UK. It's part of the Global Britain illusion.