[voksveta]

You can get 600 mins with the £35 a month contract.

Lets say you go on the £35 24 month contract. You have to pay £89 for the phone so thats:

35*24+89 = £929

In your case, over 2 years you'd be paying:

20*24+450 = £930

Assuming you're going to use the phone for at least 2 years (£450 is a lot of money for something you're going to use for less than that), I don't see how that's a great deal...

Some people prefer to buy a phone outright, others are running out of contract. It is more for people thinking of leaving o2 as they are paying £45! Think about it