My understanding is that they won't do a reserve against your credit card for the deposit. When I got a contract Orange phone in the UK, they required a 250 pound deposit for domestic only. They wanted employer verification for international roaming. The fact that I wasn't on the electoral role was also a big deal.

HSBC gave me a credit card based on local references, an introductory letter (verified) from my American bank, and a large wire transfer.

In sum, it was a big deal to get a UK contract phone. If I had to do it again, I wouldn't do it. If you have a UK debit card, you can set your prepaid to automatic top up.