Last spring I was very concerned about my son's lethargy and constant tiredness. He almost had the symptoms of M.E. I suggested a visit to his GP, as the stay-at-home parent of a baby and a 3 year old he needs every ounce of energy.
After a series of blood tests, he too was diagnosed with extremely low Vit D in his system,, his GP hadn't seen such a low count for years. As a child he might have contracted rickets!! He was given a concentrated injection, medication and good advice about sitting in the sun periodically. He has done a complete turn-around, so much so he is working from home, caring for the children and living a normal life.
Retiring and living frugally in money from downsizing after years of stress

... Can't sort out my As from my Ds.
So on mainland UK by now one has to take the pills if levels are low. And goodness knows it has not been a summer for basking in the sun has it.