In dry spells, you can treat timber structures, including garden furniture, with wood preservative and stain. Only do this in a well-ventilated space, to reduce the risks to your lungs and eyes. Make sure you use appropriate products - creosote (for example) is no longer legal.
Check and repair pergolas and arches if needed.
Wash out your old pots; use a product such as Citrox, to reduce bacterial and fungal spores without causing toxicity to plants.
Pipework and power
Now is a good time to consider installing garden lighting, lights and power points to sheds and outhouses, so that you can garden on wet days and in the evenings.
Install drainage and water pipes.
Ensure all stand pipes and irrigation lines are drained to avoid damage from water freezing in them. Put lagging around outdoor taps to prevent freezing and to enable use throughout the winter.
Paths and projects
Garden contractors are often short of work in winter and therefore available to do major tasks such as paving, fence building and pond digging.
Take action to remove algae from paths if they start to become slippery, beware of using too much salt as this could damage your plants.
Make a cold frame as a winter DIY project - you can save lots of money, as good cold frames are very expensive to buy.