The Busy Lizzies are dead, pull them up and compost them. You have a choice with the Dahlias. Some people leave them in the ground with a bit of cover on them - some extra compost, dead bracken or what-have-you, and cross their fingers. Others cut off the top growth to about 4", dig them up, hang them upside down to dry off a bit, and store them frost free in dry compost or sand. The choice is yours, depending on how cold you think it will get in your garden and how much effort you want to put in. I've lost tubers both ways - there are no guarantees with either method.
Frost or mould, the choice is yours! :-(

I left my dahlias in the ground through last winter and they survived.
My busy lizzies and nasturtiums which were blooming well, were also slaughtered by last night's frost. that's the way it goes!

Funny how up here in Northumberland, we haven't had a frost yet? Dahlia's near me still in full flow.