If an item costs £10 excluding VAT, and you buy it from a VAT registered company, you will have to pay £12. You sell it for any price you like, and you don't add VAT.
Let's say you set the price at £15 and sell 1000 items for £15. You take £15,000, you spent £12,000, you make £3,000 profit in your pocket and you'll pay taxes according to your profits (£3,000). It doesn't really matter that VAT was involved, it just affects the price that you pay.
If you mostly trade with private customers and not with companies, being not VAT registered is a good idea, since by not having to add VAT you can keep your prices lower. It's different if you trade mostly with VAT-registered companies. In that example, if private customers are willing to pay £15 but not more, if you were VAT registered, you couldn't just charge £15 + VAT = £18, because your customers would stop buying. So you'd have to charge £12.50 + VAT = £15 and make less money.
But if you sell to a company, it doesn't make a difference to them if they pay £15 without VAT or £15 + VAT = £18. You have to send the VAT to HMRC, but you can subtract the £2,000 that you paid yourself, so you make £2,000 more profit.