I live in the Midlands and I wouldn't risk planting them out yet. I always wait until the beginning of June
But i do make up my tubs and baskets once i've got the plug plants but i keep them indoors during April but once May has arrived i start to put them out during the day when the weather isn't tto bad. This way it's meant to help harden the plants off.
But having said all that, if there is no frost forcast, i cant see what harm it can do to leave them out in the garden/
Maybe i just like to play safe.
Hope this helps.