Here is my first product that I think is going to sell well in the UK that I have not seen in shops yet. It's called the "iPhone 4 Horn" It's a simple piece that slots on to the bottom of your iPhone for added acoustics and comes in many colours to choose from. The price for one unit works out to £4.26 delivered to the UK.
If you order 100 units the price is £253.22 which works out as £2.50ish per unit. I would believe these would retail for £9.99 so thats a tidy profit.
You can buy these from Alibaba here.