If feelings of depression, or just a lack of "get up and go" only happen in the depths of winter then SAD obviously should be considered.
However, if you get depressions and lack of energy frequently, just more _intense_ during winter, then it might be worth asking your GP to consider a (simple) blood test for thyroid gland function.
This can usually be corrected by a pill per day of levothyroxine.
and, quote from bmj.com, "patients with hypothyroidism are eligible for a medical exemption certificate for prescription charges (FP92)"
It certainly works wonders in countering "Black Dogs"