Here are some of the best flowering shrubs for pollinators in the UK during the month of September:

1. Butterfly Bush (Buddleja davidii): Butterfly bush is a popular shrub that produces long spikes of fragrant, purple or white flowers that are very attractive to bees, butterflies, and other pollinators. The flowers are a great source of nectar.
2. Rose of Sharon (Hibiscus syriacus): Rose of Sharon produces large, showy flowers in shades of pink, white, and blue that are very attractive to bees, butterflies, and hummingbirds. The flowers are a great source of nectar and pollen.
3. Russian Sage (Perovskia atriplicifolia): Russian sage produces spikes of small, lavender-blue flowers that are very attractive to bees and butterflies. The flowers are a great source of nectar and pollen.
4. Abelia (Abelia grandiflora): Abelia produces clusters of small, fragrant, pink or white flowers that are very attractive to bees and butterflies. The flowers are a great source of nectar.
5. Fuchsia (Fuchsia spp.): Fuchsia produces showy, pendant flowers in shades of pink, purple, and red that are very attractive to hummingbirds and other pollinators. The flowers are a great source of nectar.
6. Bluebeard (Caryopteris x clandonensis): Bluebeard produces spikes of small, blue or purple flowers that are very attractive to bees and butterflies. The flowers are a great source of nectar.
7. Beautyberry (Callicarpa spp.): Beautyberry produces clusters of small, purple or white flowers that are very attractive to bees and butterflies. The flowers are a great source of nectar.
8. Spirea (Spiraea spp.): Spirea produces clusters of small, pink or white flowers that are very attractive to bees and butterflies. The flowers are a great source of nectar.

By planting these and other flowering shrubs in your garden, you can help support local populations of bees, butterflies, and other important pollinators throughout the autumn months.