Based on your post, it would seem that the CP City is the only hotel that consistently offers upgrades with lounge acess in London (not being an RA, I'm not considering the IC).
What about the CP Shoreditch, though ? Does anyone have upgrade experience there ? Is the lounge worth it ?
I've also been reading the "Please post your Platinum experience" thread but, being unable to search (there may be a way, but I don't know it), the process is rather tedious...
Edit : I have since found one review :
http://www.flyertalk.com/forum/9387425-post537.html