Sim Free Samsung Galaxy S7 Refurbished for £389.99 at Mobilephonedirect.
These are generally customer returned products within 14 days of purchase. Hope for this price you can lay your hands on currently the best Android phone in the market.
Similar kind of deal available at O2 PAYG but that is marked as "Almost Perfect".
'New Other' tag S7 phones on Ebay are going for £430+ or so. So looks like for £389.99 its not a bad deal to try.
You can still return the product within 14 days if you are not happy.