Jake, I would not worry too much because at this time of year a lot of ground needs to be clear in readiness for spring plantings. Perhaps you missed a few sowings last year such as purple sprouting broccoli, overwintering caulis and savoy cabbage in June, then salads such as land cress, chervil, sorrel, spinach and wild rocket in July/August, lettuce in August and then lambs lettuce, spring cabbage and spring onions in late August. Also garlic in October and broad beans in November/December.
The last four are for hungry gap time of April/May which could be hungrier than usual this year because of severe cold continuing (-7C here as I write). A fast sowing to fill some space now is peas to crop for shoots, sow a tall variety undercover, on windowsill etc to plant out in April under a fleece cover. Peas stand a lot of frost. Spinach and lettuce too. I cover them all after planting
At this time of year I do not cover beds with fleece before planting as the warming effect is minimal compared to the main benefit of protection to plants after planting. One of fleeces main benefits is keeping wind off leaves and on that note I wish I had covered some outdoor salads which have been dessicated by the exceptionally cold and strong north-east winds of recent days, Monday especially.