I live in the South and I dont plant out until the end of May after hardening them off for a week or so. If you plant out keep an eye on the weather and protect with fleece or cloches if frost beckons. Its the cold winds at this tme of year which knock them back too and check their growth.
I'm with scroggin; end of May for me. I'm in the midlands. in my experience if you put them in now they seem to grow much slower than if you put them in when it's a tad warmer. In the past I've been left with little plants that went in earlier than ones that I put in a little later. Also of course, much less risk of frost.
We had a frost warning last night in Wales. I am in West Wales and the latest frost I can remember in 30 years here was the night of 4 June. Lots of people had to re-sow their runner beans.
I'm in the NW and planting out at the end of May. After nurturing the plants from seed leaving them a couple more weeks makes sense I'll be keeping some plants in reserve though as I've more than needed.
You can stagger planting out, plant some at the end of May and further plants mid June.