General on plot
Digging over now if there is a fine day will also have the benefit of exposing pests in the soil which will be eaten by friendly birds.
As you harvest brassicas, dig up the stems and turn the ground over. Because the compost heap will be cold and slow at this time of year, you can always bury these in the bottom of a trench along with some kitchen waste to prepare for the runner beans later in the year.
Leeks may be ready to harvest. Parsnips and swedes in the ground can come up when you are ready.
The cabbage family may be providing some sustenance and beet leaves, perpetual spinach and chards could be available.
Sowing & Planting
January is not the month for outdoor planting although you may be able to make use of your greenhouse, especially if you can keep it frost free with heaters.
Do not prune stone fruits like cherry and damson, although you can still prune apples and pears etc.
Remove any mummified fruits hanging or on the ground under the trees as these will be harbouring pests.
Since it is likely the weather will be keeping you indoors, you could browse through the seed catalogues or take a look at the seed merchant’s web sites and decide what to try this year.
Although you probably have your favourite varieties, it may be an opportunity to try some new varieties and compare the performance.