As long as you keep picking the sweet peas they may just be okay, as we've had such a late start.
Cosmos is definitely easy & late flowering.
Asters can be produce pretty bunches & flower late.
A lot of my roses are still flowering in September & beyond.
I can't remember how late alliums go, they look so impressive, although do have that garlic aroma you may not want!
Don't forget the foliage, I made an enormous bouquet last week & was so glad of my eucalyptus.