Advice on my sunflower please

This an emergency blip, but it gives me an opportunity to ask for some advice.
A few days ago I blipped my sunflower seedling. It's growing apace but with torrential rain and blustery winds forecast, today it seems too fragile to me to go outside.
Any gardening guru can tell me when I should plant it out (and any other advice)? I'm in the north of England, Wharfedale, and it will go in a spot which has full sun all day when the sun is shining.