O2 Right to cancel gone wrong!!!

Chrome020

I received an email on the 11th November that said the terms of my contract are changing and that I have the right to cancel my airtime plan and either keep the device plan installment as is or pay for it upfront.

I called yesterday to get this done and made it clear I wanted to keep the device plan installments but the advisor has cancelled both! I have received a bill from O2 this morning for the device plan at just under £500, they are now saying I'll have to go onto a payment plan, the same plan they put customers on who are having financial difficulties, what sort of implications will this have on my credit file and my O2 account ?

O2 you are terrible!