BigBertha1, I feel your pain here in Kansas. I have had success (after years of experimenting and just plain backbreaking work), with various rich composts, mixed in, rather than straight manure. It's possible (tho I'd never have believed it) that your clay is more compact than ours. I have to add amendments every year as the ground seems to churn more clay up every spring. In addition, we have tons of limestone rocks under all the clay. We live on a rock covered with clay. I also have had great success by adding fishing worms. Lovely, long blooming Veronicas, Butterfly Bush, Clematis, Daisies do well here. Good luck!!!
To think that London, or anywhere else for that matter, does not belong to any one demographic