strong flowering plants in clay soil

I think you need to add a good soil improver to enable you to grow delphiniums and lupins, I did this year on new beds of clay soil and had excellent results, hostas and lavenders will grow anywhere! hope this helps. JanetH

My soil is very similar.
For flowers, Lavatera Mont Blanc, marigolds and poppies all thrived in mine. My Lavatera flowered for at least 8 weeks. Bush fuscias also seem to love my neutral clay soil
I planted a small Escallonia (apple blossom) in March which bloomed like mad and has already quadraupled in size in just 6 months.
Hypericum and Weigela also flourished.
I was also given some ceratostigma, it took a few months to take but is now spreading rapidly and giving a late season display of vibrant blue flowers

Roses would be very happy too.

www.american-natives.com/mixes/clay.asp

Roses, roses and more roses
and if you want height then make them climbing roses with old English honeysuckle!
maybe mixed with chaemoneles (which I certainly haven't spelled correctly).
My postage stamp of a garden is full of colour all summer long with a wonderful scent in the evenings and before the house was built 120 years ago this was the clay stream bed.
PS Runner beans don't object to being grown through the rose arches either.