I've had them planted in clayey soils .... 3 of my previous gardens infact and they did just fine in them.
It'll be interesting to hear what others think, but that is my experience.
This garden is a much thinner soil and the one i have here is doing fine too !!!

Yes, they are happy on clay soils. There are lots at Wakehurst Place (Kew in the country) and that's on Wealden clay.