I take B12 and D cos I have a proven significant deficiency, which is a little different to most UK residents - OK - most are short of vitamin D.
B12 is available from HOlland and Barrett and they often have good offers. I would take 100 mcg per day (I actually inject 1000 mcg every three months). Metformin decreases the absorption of B12, so extra oral B12 is probably a good idea.
Vitamin D comes in several sorts. D2 / ergocalciferol is not as good as D3 / cholecalciferol. It is a fat soluble vitamin, so I take capsules with the D3 dissolved in oil. It is tempting to just take a big dose now and then, but this actually increases rather than decreases the risk of fractures and falls. SO to keep healthy, 800 units each day. There is some evidence that D3 improves insulin sensitivity and D3 lack is associated with neuropathy.