The weather can turn at any point so you will be limited as to what you can do on the allotment. Choose your days, and keep digging to improve soil and cover to warm up. For detailed advice try out the National Allotment Society.
Sow seeds now particularly if you have a greenhouse, but if not, somewhere indoors that is light at warm like a window ledge will do just as good. Shallots and garlic can be planted out.
If you have them, start chitting seed potatoes. If you haven’t, they are available now so what’s stopping you!