The Rapha super lightweight is nice, and I especially like the blue/white ombré. I wouldn’t say it’s that much lighter than other decent synthetic jerseys though, but of course much cooler than Sportwool. If you want a really cool jersey then the pro team climbers jersey is the one. If it’s mostly just something a bit lighter than Merino for UK summers then it’s worth looking at their Core range if you want something a bit cheaper and more understated.
I also really like the Café du Cycliste Fleurette, which is designed for hot weather. If you want something high end, but a bit different to the ubiquitous Castelli try looking at brands like Isadore Apparel, Cafe du Cycliste, Void, Twin Six or Attaquer. Also worth a look at some of these shops:
weareomnium.cc
alwaysriding.co.uk
victorandliberty.com
girocycles.com
thevelohouse.com