Even a newspaper cover helps. Of course, it all blows away even though frost and wind aren't supposed to happen...

I (urban, pretty much same latitude) usually put stuff out any time from mid-May, depending on the forecast. Horrible cloudy days are better than nice sunny ones, as sunny days mean high pressure, clear skies and a greater risk of frost. My old gardening neighbour used to say the first week in June, but that was before global warming. I've never lost runner beans planted out at Spring bank Holiday: the last Saturday in May was our date to plant out regardless.

Good growing!