I'm living in the UK, in my early twenties, white European and I need some advice on how much intake of Vitamin-D I need (IU)

I'm reasonably healthy, exercise 4-5 times a week, don't drink nor smoke, eat clean (salmon, eggs, milk, tuna, milk etc which considerably have higher Vit-D content so do I even need Vit-D?) and sleep kinda well (still working on it) but I've noticed I get slightly more agitated and depressed when winter looms over.

Could somebody perhaps link me to some reputable sources to purchase Vit-D, whether I should get a blood test first, and whether I should be taking any other supplements along with Vit-D to ensure its effectiveness. Also the difference between Vit-D2, D3 and B12, or if there others to care about.

Thanks