this is a perennial question. it is very hit and miss with the British weather. Do you know what it is you are planning to growing? if annuals then a succession of seed sowings every week for a month will give a chance of flowers when you want them. perennials are more tricky. some in pots outside, in a warmer place to bring them on or a cooler place to hold them back.

welcome to GoY too :o)

It might be worth having a word with your florist too, just in case - forewarned is forearmed and you cannnever trust British weather a hundred percent - one good hard rainstorm at the wrong time might mean that a backup would be useful. If you are stuck for ideas there's a site on google that gives hints for what's in season each month - put in Flowers August.