Like Daffy654 I use Healthspan. They are reasonably priced and they deliver in packaging that fits through a letterbox. You also get loyaly discounts everytime you use them.
I use Healthspan also. there is also an American site-iherb-that I've heard is excellent for price...ships internationally,prices are less than UK sites.
It's not so much the distributor, it's the make of vitamin. Solgar are probably the best you can find. My sister used to work in Holland and Barrett and most of their pills and potions are fairly weak in comparison- ie- cheaper and easier to sell to Mr. Onabudget.