Although Blood orange trees are evergreen winter leaf drop on Orange is quite normal at this time of year because of the lower light levels in Northern Europe in the winter. However it could also be the plant moving nutrients from older leaves to newer leaves this can be down to the lack of food i.e nutrients or an incorrect soil or compost pH level Ideally you need to check the soil or composts pH level with a home testing kit, organges are very sensitive to pH they need the compost or the soil they grow in to be slighty acid i.e pH 6.0 to 6.25.
The pH level is often changed over time by watering with hard water or the action of plant feeds breaking down within the soil or compost