I’ll either get the iPhone 6s Plus or the iPhone 7. They’re both the same price new but the 6s is old and the 7 only has a 4.7inch screen which is small however it will fit in my small hands better. I’m siding with the 7
Pretty much all refurb phones come with a 12 month warranty if you buy from a proper refurb store (i.e. not some random on eBay).
If you are considering a 4.7" iPhone I’d wait until the new iPhone SE gets released. The rumored size is 4.7" and the rumored US price is $399, which is lower than current iPhone 8 price. UK price might end up in your price range, but if not it will probably have the effect of drastically reducing the price on any new iPhone 7 and iPhone 8 models that stores still have in their stock.