But you'll have still paid however much (£416.95 minimum) for leasing (effectively) the previous phones with nothing tangible to show for it. It's still better to buy outright in the first place and sell it to fund the next model.
Apple are relying on people doing this to make money. They make nothing extra from people who pay the full 20 months, and lose a little (probably) from people who upgrade every year. People who pay more than half the value of a phone, and then give that phone back for nothing (for Apple to then sell as refurbished or whatever) AND agree to pay full price for another new phone... Cha-ching!
I don't think you understand how it works - the end result is the same whether you do this or buy outright and sell privately to help fund the next phone.