This is also known as seasonal affective depression (SAD).
During winter, lack of sunlight can cause people (particularly dark skinned people) to feel de-energised and depressed. Vitamin D3 has been recommended to help. Although vitamin D supplementation does support the immune system, clinical research has shown that it does not help with SAD.
The treatment for SAD is to get outside in the daylight as much as possible and to use a sunlamp such as HappyLight Lucent (www.verilux.com, LED Energy Lamp, Model:VT22). This model has a USA plug, so that a USA-UK adaptor is needed. An adaptor plug (e.g. Go, 315Z, Ref.554, BS 8546) with a flat insert face (i.e. without a plug insertion indent) is suitable.