i would look through the plant lists in a book such as Hilliers manual of trees and shrubs ( in most libraries ) and look at various willow trees under the name Salix . not all willows are as badly behaved as the hugh weeping one you see around (Salix babylonica ? ) ... some are quite managable and produce those catkins in spring . willow would cope with the soil conditions you describe.

sourcing of plants is often best done through mail order nurseries rather than garden centres once you ve read a few lists of descriptions , many nurseries will send quite detailed catalogues and reading them can be fun and educational...more fun than traipsing round garden centres seeing the same old stuff because garden centers generally only stock things that sell and the british public will only buy plants they know .

this isn t a recommendation for Hilliers by the way.