Vitamin D and Sunlight (in the UK)

I think most people know that the body produces vitamin D as a result of exposure to sunlight. However what I hadn't realised, until I listened to Rhonda Patrick on Ben Coomber Radio this week, was that during the winter months the sun in the UK isn't strong enough for our skin to produce vitamin D (or more specifically the level of UVB radiation is insufficient). It also seems unclear what a recommended, safe level of vitamin D supplementation might be since the amount required depends on so many factors (sun exposure, skin pigmentation, age, etc.). Rhonda states in the podcast that if you have high levels of body fat this may also increase your vitamin D requirements since the storage of the vitamin in the fatty tissue leads to a reduced circulating concentration. A Vitamin D deficiency test is available relatively cheaply (£30 at Ben Coomber's lab) and could be worth checking out, particularly if you're in one of the groups at increased risk (people with darker skin, the elderly etc.).

References:
Rhonda Patrick on Ben Coomber Radio podcast:
https://www.facebook.com/permalink.php?id=245018777868&story_fbid=10152305508207869

NHS Choices article on Vitamin D & sunlight:
http://www.nhs.uk/Livewell/Summerhealth/Pages/vitamin-D-sunlight.aspx

Link between Obesity & Vitamin D deficiency:
http://www.ucl.ac.uk/news/news-articles/1302/05022013-Obeseity-leads-to-vitamin-D-deficiency-Hypponen

Vitamin D testing:
http://bodytypenutrition.co.uk/lab/