You could probably legally treat the one which was cancelled but delivered as an unsolicited gift and keep it, in fairness.

Incorrect assumption. Before October 2006 you could change down after 3 months, from October 2006 it's after 11 months. At least, that's how I understand it. However, some people have had some success in getting their tariff reduced earlier based on inconsistent statements from T-Mobile, see Changing to a lower price plan - 3G forum : your 3G community and resource.

If you log in to your online account you can take payment holidays and reduce your monthly DD amount etc.

They don't have to provide you with a default letter, it's not a credit agreement.

That's not entirely true. You can agree to DD mandates online, and it's binding.

http://www.moneyclaim.gov.uk

Mail-in rebates are extremely common in the US...

Actually it's normally quite the opposite. There's often a 14 day cooling off period for you to test it in areas that are important to you, after that you're SOL.

Well, that might be the case but it's irrelevant.

I'm not convinced that it's completely gone for retail, but at any rate the point is that it'd be a violation of the contract the retail shop has with the person provisioning their merchant account or VISA etc, not with the customer.

In fairness, you can top up with a debit or credit card using an automated telephone system on most (all?) networks now... My (18-month) contract is about £25 a month, including unlimited internet access on my phone and enough texts/minutes for me to basically not have to worry too much about whether I should avoid using my phone because it's expensive. The phone was only about £10, compared with a SIM-free price at the time of purchase of £300 (it's now £250, but you can get them on eBay for about £90). Basically, my point is that it's stupid to make generalisations.

It's not a debt under the CCA. See http://www.consumeractiongroup.co.uk/forum/telecoms-mobile-fixed-broadband/70464-mobile-phone-companies-consumer.html If you send that letter, they will laugh at you and tell you where to get off.

No, there isn't. Any such term would be in the merchant agreement between the company and the payment processor. VISA etc are pushing quite hard to make it so that shops don't charge more for paying by card, and don't have minimum transaction limits etc, but I don't know what the situation is with regards to billing for telcos.

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Actually, I believe Vodafone are the only major network that require you to have a PAC at sign-up time. See Vodafone Mobile Phone Shop and various third party shops like Bring your number - Number Porting