I did try swapping servers - I tried the Asia server - and this worked for me.
To be clear - 2 days ago I deleted the cache and then let them game restart - initially it showed all Coop characters as lvl 1, but i quit SC2 and the launcher, and when i went back in, my character levels were what they should be, but no character in the process of levelling showed any progress on the circular XP meter. I tried again last night by swapping servers. I got spammed by all my achievements and then found I was looking at a total sweep of level 1 characters - again I quit SC2 and the launcher but when i got back in - still level 1. So I went back to the EU server and found that all characters were back in the process of levelling, and when I played I received XP as I should.
Raynor, Artanis, Zagara, Dehaka are all in progress at lvl 9 or greater, Fenix is lvl 1 as is Tychus - Swann, Nova, Han & Horner blah blah are max level and appeared unaffected.
All working.
I’m guessing from the discussions, that the cache does need to be cleared, but some element outside of the cache folder got reset when i changed servers and applied the last step of my fix.
I can’t promise this will work for everyone, but I know how frustrating it is, so I’ve taken the time to detail what i did. Good luck all 