As featured on The Food Programme this afternoon (R4: www.bbc.co.uk/programmes/b00t66nr#synopsis )
The UK guidelines are far lower than those in most other northern (i.e. fewer hours of daylight and sun) countries. Apparently, it's a particular problem for women (esp. in pregnancy) and babies/small children, and ethnic minorities with darker skintones. The melatonin in the latter's skin offers protection against UV light that, whilst beneficial in sunnier locations, can lead to a deficiency in a northerly country like the UK.
My Dutch friend has given her children vit D drops since they were tiny: I have asked a GP, HV and pharmacist and told there was no need at all. The UK has apparently been very slow to react to the issue.
The best sources are oily fish, meat and egg yolk. If you have a darker skin tone and/or are vegetarian (or eat low quantities of the above sources), you should consider supplements.
Conditions triggered / exacerbated by Vit D deficiency include multiple sclerosis, muscle wastage, type A diabetes and loss of bone density (I assume that's Ricketts).
Who knew???
Apologies for any errors: working mainly from memory.