Here's how it works (based upon you currently having 30% of your turnover as profit:
You currently pay £70 (including VAT) for your stock. You sell it on at £100, making £30 profit.
Once you're registered for VAT, you'll be able to reclaim the VAT element on your stock purchases. That's £11.67, so your purchase will have actually only cost you £58.33. In order to carry on making £30 profit, you now have to sell your goods for £88.33, but you have to charge VAT on top, making your new price £106.00
If your customers won't stand that price rise, you can continue selling at £100, which will actually be £83.33 for you and £16.67 for the tax man. So your profit will then be £25, instead of £30.
Chris