The government is spending a lot of money (some of it EU money) on eradicating Himalayan Balsam, giant hogweed, parrot's feather, floating pennywort and New Zealand pygmyweed which are very invasive and smother other species. Some garden and aquatic centres still sell these. The RHS has a good guide called Plantlife and your region's Wildlife Trust should be able to help you to source native species.
Just be careful what natives you choose. Some are just too big for a garden pond. For example native irises are beautiful but rampage about and take over.