If you are not bothered by the fact that your Apple iPhone 4 smartphone could be refurbished model, then consider one from Quick Post Phones, one of the most prolific smartphone seller on eBay.
It sells the iPhone 4 (locked on Orange) for £250 and with three months warranty. You can upgrade to one year warranty for an additional £54 and to two years for £80.
According to the seller, the smartphone has been “updated with the latest software, with brand new front and back covers fitted. [It also] comes complete with sim tray pin, brand new charger and warranty.”
The iPhone 4 was the one which introduced the brand new “boxy” chassis for the iPhone and kickstarted the Antennagate. It was also the one which unveiled Apple’s now signature Retina Display.
Other features worth mentioning, a 5-megapixel camera, 16GB onboard storage, Apple A4 SoC, iOS 6, a 1,420mAh battery, a 3.5in 960 x 540 pixel display, a front facing camera and probably the best Apple chassis ever (even better than the iPhone 5 IMHO).
It's worth noting that Orange sells a brand new Apple iPhone 4 smartphone (the one with 8GB) for £299.99 plus a compulsory £10 top up.