I would highlight that every week through the year in the UK thousands of guys and Girls are out camping, the majority are mountaineers, and backpackers and tend to head for the main activity centres, for such activities: Lake district, Peak district and Aviemore etc. For the cycle tourer in the winter in the UK I would recommend staying in similar areas.
In these areas there are plenty of all year camp-sites, normally with a hostelry near to provide evening entertainment, out of those and similar areas, camp-sites become few and far between during the Winter period, and one must resort to wild camping. The advantage because of the early dusk and late dawn, it is easier to camp without upsetting too many people.
When I have been out touring during the Winter Months, I have often found a Pub, looked to see if it has a nice beer garden and asked if I can camp in the Garden, in most cases if I have chosen right then I have been given the OK. if that failed I tended to look for a suitable camping spot, I tend to look for a bridle way joining the road, as I have often found that there is often room in places alongside the bridle way and sufficiently far from the road to pitch the tent.
Keeping warm is the main problem, and the most dangerous problem, all too easy to suffer the effect of hypothermia, the main point to watch is that a day,s cycling can leave you chilled, then if you do not have a sufficiently warm sleeping bag, then during the night the chilling effect can continue, and it sneaks up on you, until the effects of hypothermia take effect and you don't know it.
Willems and Satnavsaysstraighton Postings are very good advice, so I will not say anymore.