Last winter I planted Spring Cabbage plants in deep trays.
Covered with fleece when it was really cold.
They grew in February when the sun came out again.
But did not heart up.

Its a matter of experimentation. Polestyrene packaging
keeps them warm. Save some to put under the boxes
and make 'walls' around them. Bubble wrap sheeting fixed
to the walls to create double glazing helps to keep a few
degrees of daytime warmth in at night.
Its also a question of sufficient light, as plants need light
every day to grow.

Just a bit of fun really for those who enjoy a challenge.

Onions, garlic, carrots, Chard, lettuce, raddish, spinach, and i'd even consider planting some potatoes now in tubs ready for xmas