
hi Fiona,
i think there should be plenty of plants that could be in flower by June. If you want to fill yor pots just for the summer you could sow seeds of hardy annuals like cornflowers, love-in-a-mist, marigolds or nasturtium straight into the compost in early April. Later on, in late May, you could plant in some summer bedding plants like petunias or lobelia that will quickly start to flower and go through the summer. you could grow shrubs in pots as long as the pots are pretty big and in the autumn you can plant some bulbs for spring.
whatever you do, the pots dry out pretty quick in summer, so remember to check them and water frequently. you will also need to add a fertiliser for really good flowering. i would ask at a garden centre so that you can find what would suit you the best. check out the gardening books in your local library for more ideas.