Not done the one in Kent, but I found Constitution Hill to be not too difficult really. Think I may have been on a 21 going up there.
Be a bugger in the wet though !
Try the Paterberg or Koppenberg in Flanders for a proper cobbled climb! A lot more bumpy a road than the one in Swansea
Had a crack at Constitution Hill in Swansea when visiting friends in April. Only 300m but at 18% all the way and cobbles it's pretty brutal.
There's Downe Road in Kent too, 25 or 30% at it's steepest.