You can just buy another phone from the Apple store (at the normal price for your tariff), and get your O2 contract moved over to the new phone. It will trigger a new 18 month contract though, but you don't have to pay for the old one.
I think Apple impose those conditions on all iPhone service providers.
The various iPhone insurance policies are so expensive compared to buying a new one that it's only worth insuring it if you're worried about the cost of calls if someone gets hold of it. So long as you manage not to lose/break/have it stolen in the first 9 months or so (from memory - looked at it briefly before I went on holiday recently!), the insurance will cost more overall...
Watch out for some of the 3rd party iPhone insurance policies - they aren't new-for-old. You'll get a refurb unit ("of equivalent age and condition" it said in the T&Cs of one I looked at).
Jim