Well the green's taken care of by leaves - for white flowers, there's white trailing lobelia, might be an upright one too, not sure, and Sapphire is blue with a white eye. Bacopa comes in white, white geranium/pelargonium, Cosmos bipinnatus 'purity', Begonia comes in white, Fuchsia Delta's Sara is white with blue, pansies come in proper blue.

thank you Bamboo, will it be to early for petunias, you know I have had them in pots and baskets for years but for the life of me I can not remember when they are at there best ?

Perhaps some of the daisy family?

No, Petunias can go out end of May along with all other bedding - there is a white version, but there isn't a true blue, they're more purple. There's also the surfinia trailing petunias, also available in white. Also Calibrachoa, which used to be called Million Bells, like small flowered trailing petunias - there's a white version with a dark purple area at the bottom inside of the flower.

Last week of June is a good time for annual bedding - should all be fairly well grown on and flowering away merrily by then, but if the weather's right, you can plant it all mid May onwards, especially in the south or west, not so much up north, not sure where you are.

if they are not close to flowering you can always bring them into shelter to encourage flowering.