Given the record for past British winter ‘s there is no doubt that it will be a cold wet one again this year, which means there is a change in routine for most horses.  The weather normally forces yards to cut down and have limited grazing to look after the land so that it can provide adequate grazing throughout the year.
Your horse will spend more time in the stable, and you need to feed accordingly. People often have more problems as the horses come out fresh, especially with a blast of cold winter wind and a fresh clip.

Horses are grazers, so its important to give them a steady consistent amount of hay or haylage as standing for long periods of time then being fed can often cause gut problems. It is also important to study the hard feed as well, and cut down on protein and energy feeds until your horse gets used to the new routine.
With the dark nights creeping in most people will find it hard to ride, so your horse could come out full of energy. With youngsters, be safe and put them on the lunge in tack for 10 minutes until you take the edge off them, this will help to warm them up too.
If your hacking on the road I recommend hi-viz wear, as I have met riders on narrow lanes and its easy to blend into the back ground, so a normal driver wouldnt be as cautious.

Make sure you warm your horse up appropriately, I use exercise blankets on extremly cold days, its much better for older horses to stop them being cold and stiff.

Allow extra time to travel to shows, and extra time to warm your horse up, its equally as important to cool your horse off.
Know how to care for a horse that is ridden during winter.

Riding in winter brings up two issues – preparing the horse adequately for the ride, and cooling the hot horse down safely in the cold after the ride, and the following steps will deal with preparation, riding, and post-riding. Be aware that there is no temperature bar to riding a horse, provided your horse has adapted to the temperature (which requires regular outings) and it's not too cold for you. Fierce cold winds are not ideal for riding, nor are severe thunderstorms or blizzards, so use your common sense when judging the riding conditions. It is extremely important to take good care of your horse's hooves in the winter, because the mud can cause serious thrush, Aim for four to six hours weekly of riding during winter. This is especially important if you're a competition rider, as well as ensuring that young horses get used to the work required of them.

Plan well ahead. Winter means longer driving times, longer preparation times, and shorter days. Be aware of the time constraints affecting your ability to ride your horse and care for him.