I think £300 is very reasonable, but as duncs has pointed out, the iPhone 5 or whatever it's called is due to be released this week, the 2nd hand price for the iPhone4 is expected to be lower as most geeks will be getting rid of their iPhone 4. Having said that, if you unlock your phone before selling it, it might be more attractive. Are you still on a contract with O2? If not, just call them and have it unlocked for free.
I think £300 is very reasonable, but as duncs has pointed out, the iPhone 5 or whatever it's called is due to be released this week, the 2nd hand price for the iPhone4 is expected to be lower as most geeks will be getting rid of their iPhone 4. Having said that, if you unlock your phone before selling it, it might be more attractive. Are you still on a contract with O2? If not, just call them and have it unlocked for free.