The second hand market for phones is nothing like it use to be. If you plan on upgrading every single year, you won't come out better than the IUP, even if it's 15 months in.
So you pay 20 months total if you don’t upgrade or 11 months if you do. You also pay a £69 deposit.
11 months September to September for a 64gb Pro Max including the deposit is £772.45 paid.
You’ve now a choice of paying the last 9 months of payments which is £575.55, or handing your iPhone in for a new one.
Those prices above include AppleCare+, you can typically get it cancelled and get a refund for the remaining amount which should be half. So that’s another £99.50 from the total price of £199.
Now you’ll need to pay £476.05 to own your iPhone and sell it on.
I think it’s very unrealistic that a Pro Max will go for around £476.05 after the 12 comes out. The XS Max last year went for £700-800 when the 11 Pro came out.
The 11 Pro is going for about £800-900 right now on eBay. Sure it’s not hassle free but if you value the easy nature of IUP more than the odd £250-350 like many of the IUP users then fair enough. But you can’t say that the second hand market isn’t as good anymore and that you won’t come out better off than the IUP when the maths doesn’t quite match that.