Roses, roses,roses! Roses thrive in my heavy clay, but you do need to prepare the ground well.
Over the years I have added quantities of fine grit, garden compost and rotted manure to improve drainage and can successfully grow a wide variety of plants including clematis, especially the montanas, iris, phlox, veronica, poppy, allium, daylily, cotoneaster, hardy geraniums, sambucus nigra, etc.
In addition you can grow any number of annuals such as sweet peas, salad foods, runner beans and courgettes. I don't have much luck with root vegetables but then my main interest is in flowers!

Great list - add crocosmia and michaelmas daisies?

thank you both very much,just helping a friend bring back an overgrown garden ,grass lawns are my speciality so flowers are a bit of a mystery to me or were till now ,all the best ,John.