Hypervitaminosis D is hard to achieve _by sitting in the sun_ because then your body makes as much as it needs and then stops making any more.
It is not hard to achieve when taking supplements, because you can always stuff yourself with way more supplemental vitamin D than your body can clear up before it starts causing problems.
Source: I was prescribed vitamin D3 by my endocrinologist (it's actually officially a hormone, cholecalciferol). I had regular blood tests for a year or so (for reasons) that were monitored by my endocrinologist, then at some point she told me to stop taking the D3 vitamin supplements because the amount of D3 in my blood was reaching the upper limit of a healthy person's. I too live in the UK.
It is not hard to achieve when taking supplements, because you can always stuff yourself with way more supplemental vitamin D than your body can clear up before it starts causing problems.
Source: I was prescribed vitamin D3 by my endocrinologist (it's actually officially a hormone, cholecalciferol). I had regular blood tests for a year or so (for reasons) that were monitored by my endocrinologist, then at some point she told me to stop taking the D3 vitamin supplements because the amount of D3 in my blood was reaching the upper limit of a healthy person's. I too live in the UK.