We're more west than you, and probably higher up, so that although we are in a local frost pocket, we're still at least 350ft above the valley floor. So quite possibly have less frost problem.

The greenhouse is for cacti, for overwintering things that will be outside in the summer, and for veg that people in more favoured and less slug-ridden areas would grow outside. So once the weather has warmed up, I can just leave the doors open. I can see that you might have more of a problem.