Regarding deficiency, your GP can measure this for you for nearly no to no cost. No matter what your reasoning for taking some, it's always good to check what the baseline you're dealing with actually is before. For context, I'm from a country where low vitamin D levels are very common so I had a test done a few times throughout the year and turns out I've never been under the recommended levels.
Your high latitude health service must be better than mine. I can hardly imagine my GP's receptionist's reaction if I tried to book an appointment to check my baseline vitamin D levels!
