Mobile phone contracts

After looking into each mobile phone service, DH and I want to sign up for O2 when we arrive, to get free iPhones.. how hard is it to obtain a contract through a mobile service provider, having no credit in the UK?

Do you think O2 will flatly reject us, or is there a way to get around this? We would use pay-as-you-go but I don't think we're eligible for the free iPhone that way.

When I'd lived here for a year, I applied for a contract with Orange and had to pay a hefty deposit, which they held for 12 months.  Maybe O2 will do something similar.