Its hard to put numbers to your problem. You can ride on a 3 speed and it has been done before. You will do best if you travel light, without camping gear. I generally rate my travelling speed at 10mph whatever the bike and load combination.
This time of year, your main problem is lack of daylight. You have to be on the road by 7:00 and stop by about 4:00. Navigating strange country roads in the dark is not much fun.
Winter camping in the UK is quite difficult. Typical lowland commercial campsites are mostly closed, upland areas are very cold, wet and windy. The ground is very wet and nothing dries. You need to carry a lot more clothing and heavier duty sleeping bag etc.
Hostels are more common in Ireland than UK but YHA hostels should be available most of the way to the ferry. Using trains for part of the way is a good measure and a useful and safe bugout route.