Plants that are hardy and can be planted out without worrying about the weather, this includes primroses, viola, pansy, bellis and forget me not. I would also include spring-flowering bulbs like tete-tete and hyacinths.
I think it is important to say that they won’t grow when you plant them, it is too cold, you need to buy fully grown flowering plants.
Combine any of the spring bedding plants for an instant patio pot.
To add height to your planters use small conifers, hardy ferns or spring bulbs.

Primroses are the most popular early spring flowering plant.

Violas come into flower quicker than pansies.

Pansies have larger flowers

Bellis available in white, pink and red

Myosotis or Forget me not, often overlooked but they can provide height and the delicate flowers can be blue, pink or white.

Wallflowers can provide height and fragrance.
Hardy and often in flower they are safe to plant out
Grown on the nursery in cool bright greenhouses over the winter, they don’t like warm dark stores and can deteriorate quickly. avoid stretched or yellow-leaved plants