Flowers for an end of June wedding

I wouldn't rule any flowers out, you may be thankful for a bucket of single dahlias.
I must be blind to earwigs...I never seem to get many. The single bishops would be a good option if you don't want them...they have nowhere to hide. A few bishops tubers now would give you a good flower.

Plugs of snapdragons? They were amazing for me last year.

I've recycled my SR a catalogue

I sow lots of those varieties autumn and Spring, so I'm not sure how long each one from seed. If you have a greenhouse, plant some in the borders as you will get bigger plants? I'd check Chilterns seeds for how long each one flower from seed to be sure.
The Veronica is a perennial. I'd be looking in friends gardens now - it's surprising what you can find when you know what to look for. I think cosmos will definitely be your friend here. They are usually brimming with flowers. Why didn't she tell you last autumn? such an exciting thing to do.
Garden flowers are beautiful, for button holes ive often used poppy seed heads, maybe buy in a couple of early Chrysanthemum pot plants?
Nigella is quick from seed?