Where Would Be The Cheapest Place To Buy An iPhone?

I would like to buy an iPhone 4 Or iPhone 4S (Preferably The 4S, But Depends On The Price)
I would prefer the iPhone to be in white, But black will do me fine.
I have a rough budget of about £250-£300
I Would Prefer A New One, But A Used One Should Be Fine. (As Long As Everything Works In Its In Mint-Good Condition)
I Would Prefer It Not To Be Of eBay, As I Have Had iPhone Problems With Them Before (I Brought It, Only To Find Out It Was Stolen, The Owner Claimed It Was Unlocked And Worked Perfectly, When It Arrived, It Had A Smashed Back, Cracked And Scratched Screen, Stolen So It Was IMEI Blocked, Locked To T-Mobile, And Unresponsive Touch-Screen, So He Lied About All That, And Claimed 'It Must Have Happened in The Post', Yeah Right!)
Amazon Seems Fine, But Preferably A Official Phone Shop.
MUST Be On Payg, But I Might Decide To But A SIM-Only Contract At A Later Time

I Think Thats All The Information i Need To Tell You!

I think MobileShop is fine! But research and search about it in one of the browsers?! As I searched many of these mobileSites they claim don, t sell cellphones? Good try