Assuming you've taken care of the device, you can get way more than £300 for it. Plus, as it's already been mentioned, you can also get a partial refund on the AppleCare+. I think the following would be a more fair comparison:
Upgrade Programme -
£49 Upfront + £33.45 x 11 = £416
Buying Outright -
£599 upfront + £119 AppleCare+ - £400 sale price in one year - £59.5 AppleCare Refund = £338.5
(You can probably get more than £400 but it may take some effort.)
You can save a bit over £80 by purchasing it outright, and that's nice, but it comes with other advantages as well. The phone is yours from the very point you purchase it, whereas if you buy it via the Apple Upgrade Program you need to wait for 20 months for that to happen; and note that you don't have the option not to continue paying after the 12th payment since you're locked into a contract. You can either not upgrade and continue the payments until the phone has been paid in full, or you can upgrade (and by doing so, you'd also be re-starting the whole 20 month contract again).
The way I see it this program is just a way for Apple to get a guaranteed stream of income out of the user in exchange for removing the hassle of having to sell the phone after a year; but that small advantage has a price as well, as I mentioned above, since you could get more out of your used phone.
Yeah yours is a little closer to accurate, but like I said to get the ease of sale you'd need to price it lower, where with the upgrade program you can just walk in and trade up. Having just quickly checked the phone recycle companies, you're getting around £350-£450 depending on size for a Plus, which is what you need to compare the Apple program too I guess, as it's for ease of use.
As has been mentioned you can pay it off in full at any time with no downsides.
Not sure why people keep going on about the phone being rented, having to hand it in when upgrading etc, you really thought this would be a way to get an iPhone for about half the cost? It's exactly the same system as if you sell it before buying the new one. If you don't want to trade the phone in, just keep paying the payments for 9 months, or call up and pay out right.
Apple seemed to have worked very hard to make this as attractive and catch free as possible. If you don't fancy the upgrade, no problem it's only cost you £50 extra. Are iPhone values significantly higher than the trade in value? Just pay it off in full and sell it privately.
It will be interesting however to see how strict Apple will be come next Friday when people in the US try to upgrade, I'm wondering how many will be charged that AC+ excess to 'repair' the damaged phone they have brought in.
A quick comparison of if you'd bought a 6S+ 128Gb vs Upgrade Program for a year on a 7+ 128Gb (Chosen because their prices at new were similar, despite not being matching models)
6S+ -
£789 Purchase + £119 AC+ - £451 Envirofone - £59 AC+ Refund = £408
7+ -
£49 Upfront + £44.45*11 = £537.95
Difference - £129.95
Considering I can afford to buy it outright, or put it on a 0% card, and I wouldn't buy AC+ which makes the gap even bigger, I've talked myself out of the program.
That Envirofone price might not have yet adjusted for the 7 launch, so I'm not sure how long you'd have to be without the current phone before the new one arrives, I've never done it, but it seems pretty strong at the moment.
I think this is as close of a comparison as we can get currently, I suppose you could work it off US prices so you could have the cost of a year of the 6S on upgrade, but I think we've been brexit'd a bit on the prices for the 7 looking at them compared to 6S launch.
If the above turned out to be true, you could just pay for the phone outright and sell it privately I suppose. You'd always be £49 worse off though, if you can refund the AC+ once it's paid off.