Recommended to get carrots underway now by planting seeds in 5-inch pots and putting them on a windowsill. Early Nantes is a good variety.
At the same time the ground where you plan to grow them should be covered in order to get the soil warmer. If you make a simple cold-frame to do this you can plant them out into the ground inside when they are ready, and the frame will protect them. Broad beans and celeriac can also be started now, either in pots or root trainers. Plant them out when ready and cover with fleece.
Generally, it is a good idea to warm up the soil before planting out or sewing seed.Use plastic sheets (bubble wrap is best) or fleece. This is a good time to feed and mulch established fruit, such as rhubarb, blackcurrants, gooseberries, and raspberries. You can also “force” rhubarb by covering it (e.g. with an old dustbin) so that no light gets in.
If you have a problem with scale insects or similar pests on your fruit last year, this is a good time to apply a winter wash with a spray. Finally, if you want to increase the area covered by small bulbs like snowdrops and crocus, use a trowel to dig up a handful of bulbs from where they are thickest and re-plant them where you want them.