The sun isn't high enough in the sky to produce much if anything in the way of Vitamin D at this time of year in the UK.
I take high dose vitamin D all year round but twice as much from October through to May. I get 4000 iu soft gels from Amazon and take 1 a day in summer and 2 in winter. I also take vitamin K2 and magnesium and other supplements to ensure, as far as possible, all my vitamin and minerals are at optimum levels.
Summary below of what he (and probably you too) should be taking. It says 5000 iu in this protocol, I can't find any 5000 iu capsules any more but would say take 2 x 4000 for at least 2 months and then go to 1 through the summer. The magnesium and zinc ensure the D is absorbed and the calcium gets to where is needs to be.
Vitamin D3: Most diets do not contain adequate vitamin D3. Start by taking 5,000 IU daily of vitamin D3 oil-based, softgel supplements with or right after your breakfast that includes a healthy fat such as eggs, nuts, yogurt. After one week, increase your D3 intake to 10,000 IU a day. Enjoy direct noon sun exposure for up to 15-20 minutes a day, when possible. If you have liver or kidney issues, please consult your healthcare practitioner prior to supplementing with vitamin D3.
Vitamin K2: A vitamin K2 diet includes lots of grass-fed meat and dairy products. Since most of us are lacking a daily abundant intake of grass-fed goods, consume about 100 mcg of vitamin K2 (MK-7). The better quality K2 (MK-7) supplements contain natto, a fermented soybean, daily or several times a week. Take your K2 softgel with your D3 softgel with or right after your breakfast. (If you cannot take soy products or are taking thyroid hormone medication, we provide a link to soy-free vitamin K2 (MK-7)). Please do not take any form of vitamin K if you are on blood-thinning medication without the approval of your healthcare practitioner.
Magnesium: Magnesium-rich foods include leafy green vegetables, legumes, nuts, and seeds. A daily supplement of magnesium malate or glycinate of at least 400 -600 mg should increase your level of this essential mineral. If you prefer energy-boosting magnesium malate, take it with your D3 and K2 with or right after your breakfast. Or take your magnesium glycinate before bedtime. (The glycine in this supplement has a calming effect that should foster sleep.)