Not until mid-May, to be safe, Palamino.

I agree Spritz, here in the midlands I tend to put things out at the spring bank holiday depending on the weather

Agree with Spritz - not before middle of May at earliest & then you'll need to keep an eye on the frosts until end of May, certainly in SW Scotland we get to end of May anyway.

RHS recommendation for putting out summer bedding is end of May - I do put mine out in the middle of May here in London, but not if its a cold spring, and mine's in a southfacing warm situation. We have had overnight frost in London on 4th June before, and obviously, summer bedding is frost sensitive (though Petunias seem a bit more resistant to it).

I forgot to say that the baskets need to be 'hardened off' for a couple of weeks before they're put out, too.

Thanks very much for your answers! Very helpful!