We've all done, Christmas is over and we can feel our fingers itch. We sit in front of the fire, doze off and wake up to find our fingers doing a merry dance, poking holes in imaginary soil, popping in seed, pinching dreamy soil over the dibbed hole. We turn to our seed catalogues, make notes, measure our beds, measure them again, draw out dreamy plans, order too much from the catalogues and hit the ground running, sowing everything. Stop.
It doesn't matter how well you insulate a greenhouse (as you can see above). You can pin up your bubble wrap, roll out a heater or even create heated beds with electric heating coils in the soil but in the end the seasons can't be cheated. Seasons can be extended, bent a little but no domestic grower can create spring in the depths of winter. We can rob a few weeks but we cannot rob months. We have to accept that our dreams of seeds are just that.
You could try to sow now, using heated propagators but you may be in danger of creating coddled plants. Plants that you have hugged close, cosseted in fleece, wrapped the warmth of a paraffin heater around them but until the soil warms up you will soon find yourself running out of space under glass and more will end up on the compost heap (if you can get into them) then in your soil.