I would agree with that. You seem to be about 2 weeks ahead of us in the spring down in Dorset, but we live on a hill in Hampshire, so it can be a bit cold up here.
Well, I planted the half of the cosmos yesterday - (no will power!)...but covered them up with a bit of fleece. However, it wasn't frosty at all last night in my garden, so all is well. I think I'll keep the fleece handy overnight...and then plant the rest of the cosmos in a fortnight or so...not fleeced...and just see what happens when they start growing properly in the summer.