Complete novice, I read you post as Dove did, ie you were thinking of covering plants with black plastic instead of using fleece, fleece isn't a ground cover but protects plants already in growth and protects them either by wrapping the fleece around the plant or covering the foliage, if you see what I mean
Using black plastic to warm up soil in spring makes sense however garlic needs a cold spell to enable the cloves to split and doesn't need ground cover to protect it. If you haven't planted out your garlic yet there is probably still time to start it in modules providing you can keep it warm enough to put out shoots.
I'm not sure about onions, I wait till spring to plant those.
Jim-I watch BeechGrove, excellent gardening programme.
There's probably little you can grow at this time of year but to get an early start next year, covering your veg beds about March-April time with black plastic will warm up your soil to give you an early start, you would then need to split the plastic where you are to plant and plant out. I'm in the NW and to get an early start this year bought a frame and then covered my veg beds with a plastic tunnel.
Look through the seed catalogues and plan for next spring. Happy gardening